Like you say, there are many freerolls offered by almost all sites, but by far the best are the CC Freerolls. Much smaller fields mean there is some genuinely worthwhile prizepools, for example $20+ for 1st in the daily $100 games. You need to qualify for the platinum freerolls, but that is possible within a month or so with regular posting.
Personally, I have built a bankroll of $140 since April 2020 almost exclusively from freerolls. $140 is more than enough of a bankroll to play microstakes and I continue to play the freerolls as well.