Microgaming have HU as small as 4NL and the rake is actually pretty managable:
https://www.mpn.poker/poker-games/cash-games/rake/
I have put in a bit of volume at 4/10nl the last two weeks and can say that, given player pools the games are unquestionably beatable. I'm up about 40 bi at 4NL over approx 50 hours of play where I have not run like god the whole time and have tilted away at least 10 bi (possibly 20) during the bad stretches.