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This was my immediate thought as well. Portable launchers for the various OS’es on a tiny (just large enough to store the launchers) FAT32 partition, then a large FAT32 partition (the majority of the drive) encrypted by VeraCrypt. As long as it can read FAT32 and run VeraCrypt, it’ll be compatible. And that covers Windows, Linux, Raspberry Pi, and Mac ecosystems. It’s not as simple as just plugging it in and getting a password prompt, but it’s going to be the most compatible while still allowing for (nearly) the entire drive to be encrypted.
I have had major issues with exFAT across a variety of platforms. But I also work with a bunch of niche gear. But my point is simply that being widely compatible isn’t the same as being fully compatible. And OP was asking for the best way to reach the widest compatibility. That calls for FAT32, even if it has issues with things like file size.