Can't add music to Library or download songs

I cannot add music to my library or download songs -- they magically disappear almost instantly. I can hit the "+" icon next to a song or album, then the "cloud-arrow" icon to download said song or album, and it appears to initiate the download process, but then... it doesn't. It goes back to the "+" icon and does not add to my library or download. This issue is persistent across both my iPhone and iPad, both of which are on iOS 14.6

I tried cycling on and off the "sync library" feature, and now *none* of my library appears on my iPhone and it's stuck on the "syncing library" phase, but doesn't appear to be doing anything at all.

Is there a maximum iCloud Music Library size? I realize that my Library is extremely large (~98,000 songs), so I'm concerned that perhaps it's an issue with approaching 100,000 songs? Any help would be greatly appreciated.