On device isn’t always ideal. I don’t use immich because i don’t have a large photo library. But I do use komga. Nextcloud can sort and manage epub/pdf like komga but as poVoq said, the specialized solution is superior
This point is where on device app is not the ideal situation, for me at least. These apps exist. Tachiyomi and the resultant forks can import a local library. And frankly even a somewhat massive local library can fit on a cheap SD card
The point of the server is portability. With this I have portability across my devices. My library, reading status, metadata, etc is available on all devices. I can read a book on my ereader, close it, the status is synced. I can pick up from my laptop and the same thing occurs. I can pick up from my phone, download the book to my device, and keep reading while I’m away from home. If I wanted to I could open remote access to my server and avoid the need for downloading the books but that’s a whole thing
I don’t think it would make sense to run a server solely for this but it’s a service that doesn’t take much in terms of resources and I read a lot.
Is the windows side up to date? Most likely culprit would be a db mismatch if one side or the other is on a different version. Similarly, do you by chance have either side running a beta release? If you have plex pass you potentially have access to these
Shameless moment to plug the idea that you could consider migrating to Jellyfin if you can’t solve the issue