Convert Traditional IRA or Traditional 401K First?

46M. I plan to FIRE (maybe at 55, hopefully at 50). In the years leading up to my retirement I want to convert some of my traditional tax deferred accounts so I can pay taxes now at the 24% bracket. FYI, I have 1mm+ in traditional tax deferred accounts.

1. There is some debate over whether I should convert some now or wait until I FIRE to convert during retirement so I can do so in a lower tax bracket. I figure I should have 20+ years post FIRE to convert some each year?
2. If I do convert some this year (~50K), should I prioritize my traditional 401K or traditional IRA? I think the 401K is easier to touch before age 59.5 (e.g. rule of 55). If that's the case, converting the traditional IRA first is better?