Sounds like you need to create two partitions and format them with relevant filesystems

Non app storage needs then… My best guess then is messaging app attachments? maybe OP needs to save docs/pdfs he gets from Whatsapp/telegram. But you’re right, that seems more of a laptop/PC activity.

Immich for pictures seems like a good answer to your question. What’s files: Mostly pictures I think. Why not just keep it on phone? people love their pictures. I have more than a phones worth.

I think people generally look for a phone app / ease of use type of thing