IPad – How to sync and store data for offline access on an iPad

I am asking a question which, despite tons of Internet research and trying out many different options, I can't seem to find a solution to. I am a software engineer by the day, and a musician at night. I recently purchased an iPad with the goal of keeping my entire sheet music library on the device. I just can't figure out how to keep the music "synced" on the device! Here's what I'm looking to do:

• I have a folder (with subfolders) filled with music. I can share these using basically any protocol: web-dav, ssh, FTP, Dropbox, you name it. I primarily use rclone to move files around and it's easy for me to set up a server with whatever protocol makes sense.

• A requirement is that the data remain on my iPad offline (don't want to depend on Internet access at a gig!)

• Also a requirement is to be able to sync an entire folder, rather than selecting files individually.

• My preference is to self-host my data, rather than relying on a proprietary service. If I have to use a service, I do not want to pay a monthly fee just to be able to sync. This is why Dropbox Pro doesn't work for me – I don't mind paying a hosting provider to store my data, but don't want vendor lock-in.

• I don't mind paying for an app to do this.

How can I achieve the above? This is what I tried on the iPad:

• Dropbox: Would work beautifully, but I would have to subscribe to their Pro plan (in perpetuity!) and I'd rather pay monthly fees for a system without vendor lock-in.

• Google Drive: You have to select individual files for offline mode, not a folder.

• Nextcloud: I used to use it but found their sync to be very buggy! Maybe it has gotten better in the past 1 or 2 years.

• Amazon Drive: Does not have an offline mode.

What should I be trying that I haven't yet?