I sometimes think of it this way, too, but I include some conversions. I'm using the 401k as the "62+ bucket." I'm going to split income near retirement from Roth and 401k, to draw down the 401k at half-bracket tax rates, and treat the entire Roth as "safety margin."
It sounds like my FIRE goals may be larger, but I think I share your thinking about time. 21 years of relative freedom is more interesting to me than 14 years of total freedom and 7 more of my youngest remaining years pulling my hair out. "Barista FIRE" can also continue to take advantage of 401k matching.
The big question is your time to FIRE. If your taxable has to cover under 10 years, you could lock in CDs near 3% now and be done with it. I think the fear of a market downturn in any window over 5 years is unnecessary - I'm leaving all of my money in stocks indefinitely. Long-term, it's the best bet.
What do taxes look like...? Not much! I would let Vanguard work its magic with VTSAX for the simplest plan. Withdraw every year what you need from it. Most proceeds will be long-term capital gains, and if your needs are low (as you say), you should wind up paying very little.
What kind of asset allocations might be good...? 10 years or more, I would still do 100% stocks. I mean, I would do all stocks even at 5 years, but YMMV. CDs pay near 3% now - if not stocks, today, I would do CDs.
How should you change asset allocations as you go...? I would just choose how I wanted to set it up in the first place.
If you wanted to go conservatively, set up a 1, 2, 3, 4 year CD ladder with 4 sets of annual expenses (start them all at once) and keep one year's expenses in the bank for the current year, then leave the second half of your taxable money in stocks.
OR, just leave it all in stocks.
At year 7, if the market did well, sell and go to CDs for the remaining years. If it did poorly, leave the stocks alone to the extent you can.
Disclaimer: I would rather leave all the money in stocks rather than time the market with the "taxable bucket" this way, but if it helps you to go enjoy your life to think of it like this, the lost returns on those remaining 3 years would not likely be catastrophic.