https://github.com/facebookexperimental/hermit but it hasn't worked for me and is now unmaintained.

There's languages that support time travel debugging, like RR for GDB, or smalltalk, but no open source system wide thing like Antithesis that I know of yet.

rr can record process trees; i.e. basically any part/descendant of a process you spawn will be recorded and can be replayed (userspace CPU & memory, that is); won't record the entire OS though.

My experience with RR is that the chance of it working without hitting a missing syscall or desync is only about 50%, which is why I want a different solution that doesn't rely on the fragile syscall recording approach.

Huh. In my experience it works nearly flawlessly, certainly far above 50%. And even when there are spurious failures in replaying it's easy enough to just re-replay (though I do wish there was some way to export the current position & checkpoints with instruction-level precision to import in a fresh replay). I suppose it depends massively on the recorded program (most of mine are simple C programs, but also a decent bit of Java for JIT inspection or FFI, and I've also recorded an Electron app a couple times, and for fun Factorio)

Same, I haven't had it have too many problems but it's not perfect & missing support for io_uring is a problem (they'll add it eventually I suspect once someone ponies up the money for it).