All good
It seems to me that with #4, you are no longer maximizing your Roth conversion for 2020 because you are taking some of that tax space for the tIRA withdrawal for the kids. I think #3 is better.
Let's say you're looking to gift $10K to a son, and also plan to maximize Roth conversions as best you can.
With #3, you gift shares that have a large gain. No taxes are paid since (I'm assuming) your son can sell them at the 0% LTCG rate. You can convert an extra $10K that you wouldn't be able to with #4, so you pay the same tax on conversion or withdrawal, but now an extra $10K is in a Roth.
With #4, you withdraw the $10K from the tIRA, and that's $10K less you can convert because of the ACA subsidy income level constraint, but you have $10K more left in taxable. You pay the same tax on the withdrawal as you could the conversion. You son is in the same situation with the $10K and a new basis as he had by liquidating gifted shares with a stepped up basis.
So your son is in the same place ($10K and $0 tax liability) and you have paid the same current taxes. But with #3, you have $10K in your Roth rather than appreciated shares in taxable, so if you do need the money, you can access it tax free, so you're better off. And your son inherits $10K more in a Roth and $10K less in stepped up taxable. Even if the new inherited IRA rules take effect, he'll still have some time to let that $10K grow tax free in the Roth for a few years so he's better off.
With #4, if it turns out you'll need that 10K, you're going to have to sell appreciated shares from taxable, so you'll incur LTCG taxes on the sale. Also, any dividends that $10K throws in taxable is subject to tax, and eats into the amount you can convert under the subsidy cap. If you don't have to sell it, your son inherits money in a taxable account rather than a Roth, which is less preferable.