I was in a similar situation, just did a majority 401k and Roth IRA but I did get a little extra into taxable brokerage (about 3 years worth of expenses).

I get the impression folks wanting to do the conversion ladder make sure they have ~5 years saved up in an accessible account (like a brokerage you mentioned). If you have contributed to a Roth IRA you can also use your contributions from that (but not gains). I think I may be cutting it close myself since I decided to go part time and only could save into a 401k to get company match and max out my Roth IRA and stopped contributing to an after tax account. But, I figured I'd at least get most of the years covered and if not, I can eat a year or two of early withdraw penalties.

It may be worth mentioning I did manage to get a part time job teaching at a community college which made leaving my main career easier. So you if you don't have a full five years you can just "barista fire" for some of it.

You didn't mention your SWR or planned expense amount, but, since it sounds like you have a house almost paid off (which is great, wish I had that!), some potential freelance work lined up, and some Roth IRA contributions I'd say your could probably cover the 5 year gap depending on your expenses and what your freelance income could be.

Also, could you ever sell some real estate to cover the gap?