Convert from 401K or Regular IRA? Tax Implications?

I am thinking about converting some money into Roth to fill up my 12% bracket (retired now but pre-SS and living on after-tax savings).

I realized over the years, for my non-deductible IRA contributions, I did not consistently do that IRS form (8606?) to establish my basis. I can't get this done before year end. I know I have to eventually do this.

So, I am thinking maybe I can convert from my pre-tax 401K. Does it make a difference math-wise either way? For the same taxable dollar amount, I suppose I can convert more if I go from the non-deductible IRA once the basis is subtracted?

My 401K also has some after-tax money. I read that if you convert the after-tax money, you have to drag some pre-tax money with it based on the proportion, creating more taxable income. It seems the reverse might or might not be true (i.e. can move pre-tax money without dragging after-tax money), depending on the 401K plan?

Also, I have been making estimated tax payments every quarter without taking this into consideration. If I convert in December and just pay the additional expected income tax in the January 15 2025 Estimated Tax payment, would the IRS have an issue with it?

Thanks.