IMO, an OTM short put should be sold when you want to acquire the stock at a lower target price. If the underlying drops and you have second thoughts, either close the position or make a defensive adjustment. The latter should be done before the put gets ITM and you should sell time for intrinsic value (roll down and out for a credit). And yes, I know that you're already $5 ITM so you may have to sell a lot more time for a credit.
Which strike and expiration to roll to will depend on the strikes offered, whether there are weekly options and the implied volatility. I would prefer the nearest expiration that generates a credit or at worst, a small debit because the decay rate is higher and if some time passes, you can do this again if need be. If you roll out months, you reduce your ability to repeat this and you get time committed (many months out).
For example, a roll down (and out a week or two) to $75 would give you $5 more breathing room, lowering your purchase price if assigned.
Close my current short put@80 at a loss now, and then sell another put@$70 with the same expiry date to cover up my loss a little bit
This reduces your loss but it locks in a loss.
Sell another put@$70, and hold my existing put@$80 wait until the expiry date of the option. If the stock price is between $70 and $80, then let both of the option to expire to get my stock at a cost of $80. If the stock price is below $70, then I close my put@$80, and get the stock at $70.
If the stock is between $70 and $80, you'll be assigned on 100 shares. The cost of the equity will be $80 less the total premium received.
If below $70, you'll be assigned on 200 shares. Your cost basis will be half of ($150 less the total premium received). Now you'll have double the downside exposure.
Close my current short put @80 right now, place a buy limit order at $70 for the stock and wait for a deal.
That just realizes the loss on the short $80 put with the hope that the stock drops more and you get to buy it at $70. That's a maybe that might not happen. So that might mean just a realized put loss on a stock that you want but don't get.
Which strategy should I use? Is there other better ways to deal with this situation? Or which strategy is the most dangerous one that I should never touch?
That depends on hindsight, which none of us have. You have to decide if you are going to bite the bullet (take the loss), adjust with a roll (lowering cost basis), or possibly doubling up by averaging down (sell a $70 put as well).
I'd offer one more possibility for evaluation. If you are more concerned about the downside and you're willing to give up some/all of the upside, see if there is a near term OTM call spread that could be sold for a credit but would not lock in an upside loss (80/82.5, 80/85, 82.5/85?). With the stock at $75, that credit would likely be small but it would modestly lower cost basis.
As a made up example, suppose you initially sold the $80 put for $4 and you could get a $1 credit for the $80/85 vertical. The $1 lowers your cost basis if assigned on the put. That's $5 in premium received and a $5 risk on the call vertical which is break even if the stock rallies to $85. Is giving up the upside worth a $1? Ultimately, you have to decide which potential outcome suits you best and hope for a cooperative underlying.
I'd add that if you're selling premium without the express desire to own the underlying, consider verticals and long stock collars so that you have some built in protection against share price collapse. Yes, the net premium is smaller but the long leg prevents debacles (see last March). It has a much better risk/reward ratio and gives you more ability to defend.