I have tried Umbrella and CasaOS and they are really very similar. That said, learning to use portainer allows you to understand the underlying technology of docker much better and also allowd you to do cool things like making a stack of containers that work together and saving that stack in a single config file. The GUI of Umbrel and CasaOS are basically the same and they only differ a little in their internals, so I would say switching shouldn’t be too hard, but if you have found yourself struggling to get something working on CasaOS you will probably have the same issue on Umbrel.

The web interface with all the links to your services can be achieved with things like Homarr and you can manage storage on the back end with Open Media Vault or something similar to make things more accessible. If you can have your data files accessible on a single BTRFS or similar partition with folder level sharing then you can have files hard linked between your downloading application and your media serving application, say Deluge and Jellyfin. The files download in /share/Downloads and then on completion get linked into /share/Imports and Jellyfin can manage moving them into /share/Movies or /share/TV as needed, and the same would go for something like Subsonic or AudiobookShelf.

Personally I have a wonderful set up with AudiobookShelf which syncs to my phone ereader. I can have books for my ereader with all the metadata and it is accessible via WiFi so I can just pick a book and get the epub delivered directly, no Calibre or USB cable required. It also works nicely for the audiobook side by letting me tell a whole series to download to my phone so I can listen back to back without worrying and I can always make space if I need to by clearing things out. Oh, and I can set my default listen speed to 2.0x and that carries over on other devices, so I don’t have to constantly fuss with it.

Overall, super fun, I would recommend learning about Portainer if that appeals to you, maybe consider setting it up on an old laptop and playing with it there, and if it works out nicely then you can redo your server and get it all working nicely there.