Those big files like .m4b (b stands for book) should have chapters within it, if you open them with mpv on your pc you should be able to see them on the time bar. On Android I’ve been using Voice, it’s really well polished and shows a big chapter name so I usually remember where I was if I switch devices, even if not to the exact minute.
I figured out how to encode to a single m4b in fre:ac so I only use Voice now (or my ipod, which was the reason why I learned how to use fre:ac).
I know you asked for syncing (one day I’ll try adding the audiobook plugin to my jellyfin), but this works for me.
If you prefer a folder of files, you can use fre:ac or many other encoders/tools to split them up.
Blxter