Some people actually just started reading this on Discord using the unofficial WaniKani book club server. They are only three chapters in at this point. I’m also taking this opportunity to reread the series.
ぜんぶきみの性 (series) | L22 is literally about two main characters who switch back and forth between male and female. It can be a tad ecchi, but I’ve been able to bear that part and have enjoyed it overall. I still have to get back to it to read the last few volumes.
I’m almost certain you’re already familiar with らんま1/2 (series) | L23, but I feel like it has to be mentioned here in case anyone hasn’t heard of it yet. I’ll have to check out some of the other recommendations from the last few posts too!
Edit: just realized I didn’t actually explain anything about the series I’m recommending, whoops .
らんま1/2 is a 1987 manga by legendary manga author Takahashi Rumiko (めぞん一刻、犬夜叉、うる星やつら、MAO) where the main character turns into a girl when splashed with cold water and back to a boy when splashed/soaked in warm water. Also other people turn into other things and there’s a lot of ridiculous fighting and shenanigans and a panda.
ボクガール, has some stuff that goes well beyond fan service. (I actually appreciated the fan service parts tho, because it felt appropriate for the series… normally I dislike fan service b/c it feels disruptive or out of place). It definitely gets very ecchi tho, not always in a good way.
ぜんぶきみの性 I liked, but not as much (more serious, and some things hit too close to home), but of the two, I would more readily recommend it. It’s pretty boring for the first 2 volumes, imo.
Re-reading/watching らんま1/2, and really enjoying it (mixed feelings about the anime as an adaptation). In contrast to the other 2, Ranma isn’t really thematically about gender. It also doesn’t rely on an interfering 神様 to move things forward. I’ve been enjoying it the most of the 3, so far! enough to impulsively order a set of all 38 volumes
There isn’t any switching back and forth but 天使な小生意気 is kind of genderbender-y.
The main character was a boy who was turned into a beautiful girl when they were young. It’s a comedy, delinquenty sort of manga, but also romance, and the main character being sort of split between whether they are a boy or a girl.