In terms of how to make your decision, here are some considerations.
Comprehensive insurance often covers other perils besides collision, including fire, theft, hail or other weather damage, additional liability coverage etc. It may be worth looking at your specific policy to see what is covered. No matter what you do, make sure you have some form of personal liability coverage in case you are sued (doesn't necessarily need to be through the auto policy).
While it can make financial sense to drop comprehensive coverage once you can afford to self-insure against collision, this will only be the case if you are certain that you can set aside dedicated savings that you would only need to dip in to in the case of a collision or other major loss. For example, if you only have $5-$10,000 in the bank, and you happen to lose your job, and then the next month you happen to be hit in an accident and the car is totaled, could you afford to replace the car out of pocket?
I would recommend looking at dropping comprehensive insurance as similar to a "DNR" (do not resuscitate) order for your car, i.e. under no circumstances would you choose repair the car were it totaled or damaged. For example, if your car's exterior were badly damaged in a hail storm (but still ran fine), would you pay $500 or more to repair it, or would you simply get a new car?
Ultimately, this is going to be a judgement call based on how much financial risk you want to take on. Personally, I would continue to pay the extra $300 per year for now in order to insure a $6-8,000 asset (5% of the asset value) However, in the next few years the resale value of your car will continue to decline. If in a few years the car were worth $1,500, I would probably not pay the same $300 a year (or 20% of the asset's value). When you should make that choice depends on how many more years of service you expect to get from the car, which is a very localized question.
Hope that helps!