Thanks @ian57 for the clarification.
Kodi and Samba 1.0 could be the issue. I can't find out what smb-version my NAS (a LaCie 5big network 2) has on board. Though I don't have any issues connecting Win10, Mac Catalina or Ubuntu Focal to it, which seems strange is smb 1.0 is such a big problem.
exFat is actually good news, concerning bigger files. But even on exfat it is not possible to have multiple files with the same name but different uppercase/lowercase letters - they would still be treated as the same file! (unlike on linux filesystems for example).
So giving errors for not finding files according to different case sentitivity is simply a dumb design decision.
Perhaps I will go through all the pain again and try to set up recalbox 7.1 on my pi. Lockdown gives me plenty of time anyway. Just looking for the good (no, I don't say "positive" ) things.