Why you're leaving
I'm going to start by examining why you might want to leave. As I see it, you have a business that generates 260k in revenues every year (280k twice, but in the first two years) with 30k overhead and 230k salaries. However, if we include your salary at 100k, your salary cost goes to 330k. You're actually losing 100k a year as an opportunity cost.
Now, I can't validate a 100k salary. I'm basing this on your self-description as a senior developer and your description of the 100k developer as a senior developer. It is possible that you are overestimating your worth. But regardless, you are currently drawing zero and the cheapest other developer is 50k.
My concern here is that you seem to have 360k of work for which you are only charging 260k. Obviously that is not sustainable. Either salaries need to go down or charges need to go up. If charges go up, you might get less work. So losing a senior developer may actually help the business. Perhaps removing a 100k salary and increasing the rate will decrease the amount of work just enough for three people to cover.
Why might they want to stay?
One possibility is that with your departure, they can raise their rates and still get enough work to support three developers. Presumably you can't do that now, as you barely have enough work for four developers. Take one of the two most expensive away, and maybe that improves.
Alternately, making them owners may make them take more reasonable distributions. Currently, you pay them, then you. That's great for them but not so great for you. If they can't actually demand 100k, 80k, and 50k salaries elsewhere, then this job may be of outsized worth to them. If their worth elsewhere would be more along the lines of 70k, 50k, and 40k, that would have left 70k to pay you. Going forward, it might allow them enough room to make a profit.
Again, I can't actually say how much any of you are worth as developers. I'm throwing out numbers that make your finances work. Are they correct numbers? I have no idea.
Regardless, this gives them two ways to make a successful business without you. One would indicate that you charged to little. The other would indicate that you paid too much. Obviously, they would prefer that it is the former.
Valuing the business
You currently see the business as having no profit. This is because you start by paying the other developers a fixed amount. However, if you switch them to owners, then their salaries no longer need to be fixed. Instead of being a fixed cost, they can now become profit distributions. That's how they've been treating you for five years. They got a fixed amount while you got the remainder. When selling, you want to flip that around.
Since you are no longer working there, your salary will be zero. Since they will be owners, their fixed salary will be zero. If there is excess money, they can pay it to themselves. This changes everything. By making them owners, you free up 230k in salaries. That's the new business profit. 230k.
You could try to apply a discounted cash flow to that profit, but it would make the numbers rather high. Using a discount rate of 2.75%, I get a valuation over a million for five years. You aren't going to get that much, because they would be better off getting regular jobs.
Assuming you are correct in your evaluation of yourself as a senior developer and that you've been putting in forty hours a week at development, then you have essentially invested 460k over five years (five times your 100k salary minus the 40k you took out). In an ideal world, you'd charge that for the company. However, that's unrealistic. They would never pay it.
A more realistic amount would be 115k. That would require each of them to give up half their salary for one year. In return, they would get equivalent shares in the business: 50k shares, 40k shares, and 25k shares. Going forward they would share any profits with that distribution. Or they could pay themselves salaries, but only out of the actual money.
It would be difficult to sell to anyone but them with them as employees. The business is not making a profit if they are salaried. It only works as a business if they can retain the existing revenues while losing your development work. They will also have to replace your other duties.
Any other potential buyer is going to see a turkey. The first thing that anyone else is going to do is lay them off and replace them with cheaper or better developers. For example, a multinational development company with project managers local to you but developers in India, Southeast Asia, or Eastern Europe might be able to make money on your existing work at your existing rates.
I would not try to drag out a payment plan for the long term. A year seems manageable. They make sacrifices in that year, but afterwards they no longer need to pay you. They will likely stick out a year under those circumstances, hoping that once you are paid, things will be better. If you set that for two years, they are more likely to give up. They'll realize that there is only 230k to pay for 330k of work.
If they can pay the 115k out of savings, that's even better for you. That avoids the problem of them alienating some of your clients and losing that business. But I suspect that they won't have that much. Take as much up front as they will give and then collect the rest over the course of the year. The agreement should be that you get paid first. They can take their pay out of whatever remains, either as salary or as distributions.
Making this work
Currently the four of you are working 180 to 190 hours a week, so let's call that 185. That's them working 40 hours a week and you working 60 to 70. Without you, if they each work 60 to 65 hours a week, they can can cover the 185. For the first year, that can keep their income up without making radical changes. After that first year, if they want to get back to forty hour work weeks, they will have to raise the rate they charge or cut back on their salaries.
The only real argument for a positive valuation to them is that they don't have to look for new jobs. That's all the leverage that you have in this transaction. The way that I see it, you might get half of one year's profits out of that. Going forward they will have to find some way to charge more or work more effectively if they want to keep their income up while working normal weeks.
Financially, this is probably a bad deal for them. If they could get regular jobs with the same salary, they would almost certainly be better off. But you spent five years finding that out. You just need them to dream for one year under this plan. And as I said previously, they may get better salaries this way than they would get from regular employment.
There is an argument that this is unfair to them. They are doing a lot of work to pay you 115k. However, I think that if you look at the whole history of the company, they are still making out on the deal. They've had jobs for five years where they did only two thirds of the work but reaped almost all of the benefits. Asking them to give you half of the benefits from one year is not unfair to them. You will still be the lowest paid employee for your five years of work, even though you were doing the most work.
If your choices are zero or something less than 115k, you still might take a lower offer. Because something is probably better than nothing. But that's also the argument in favor of taking 115k rather than insisting on being made whole.
Alternative payment plans
Another answer suggests a commission plan above a certain amount of revenue. I would advise against that. The problem is that you will spend all of the up front money on legal fees writing a contract for the plan. And you have very little leverage to get the money if they balk. For example, if they replace you with a 100k project and sales manager, they might pass the revenue target but lose money after their salaries. In that case, they won't want to pay you money.
Another issue is that they might avoid hiring so as to avoid the risk of that happening. A commission is hard to manage if they are losing money. They will tend to put that payment last. That's the problem that you've been having. You've been paid last, which meant not at all.
Such a plan also constrains them. For example, if they want to merge with another company, they would have to get your permission to discharge the obligation.
Your goal should be to get paid first, before anyone else. That way it is on them to find a way to make money. By paying it all in a year out of revenue in that year, it makes it a temporary situation. They can see light at the end of the tunnel. They may not fix the rest of the problems, but at least they can pay off you and stop paying. And you can make them pay for that whole year, as you will own the company until they pay. It's simple and can be written up in a paragraph.
This commission idea does not really allow you to own the company until they pay. It commits them to a longer term deal. You won't be able to walk away until the period is over or until the business goes broke. And it doesn't give you much even if it works. And you have no influence on making it work. It's entirely up to them. If they make it work, they should get all of the benefits. They'll be the ones taking the risk.