To your last point: try UFS! I use it for cross-platform flash drives. Better than FAT32, anyway.

I guess you’re referring to Universal Flash Storage rather than Unix File System? (:

Maybe UDF?

Probably. Last time I check UDF is a format supported by the 3 major OSes out of the box.

I recall getting UDF cross compatibility required a bit of non-trivial work[0], did this improve recently?