anamethatisnt

I haven't tried Sync-in but if simple is what you want then a Synology DS225+ should get you up n running quickly.
https://www.androidpolice.com/synology-as-a-google-drive-alternative/

Which part of the seafile install was it that made you back off?
https://manual.seafile.com/latest/setup/setup_ce_by_docker/

That was an interesting read.

If you wanna own your audiobooks you might wanna check out https://libro.fm/ - they allow you to download the bought audiobook without drm.
I gladly accept tips for similar sites when it comes to buying ebooks.