Well, it took some doing, but I found good advice here: https://www.kitces.com/blog/fix-rmd-missed-forgotten-miscalculated-corrective-action-form-5329-penal...
(Written five years ago, but I presume the principles apply.)
This adviser suggests I need file only the Form 5329 for the past years, pay nothing (be sure to leave a zero on Line 54!), and request the waiver with your "reasonable cause" and actions taken to correct the forgotten RMDs -- in my case, the full distribution taken last fall and taxes paid accordingly.
This adviser also suggests the 3-year rule about IRS tax collections does not apply, because Form 5329 technically would be its own "return" on which the clock hasn't started until it's actually filed. So presumably you could have to file Form 5329 for 10 years back or more (using the 5329 form made for each of those years). I don't know if I can even find out what happened before 2020, plus I'll have to estimate the required RMD for 2021 and 2022, but I just may make a good-faith effort for those two years and leave it at that.
