@Moon nextcloud imo. Setup might be a bit of a pain but after that it just werks, everything is available everywhere. There's a good sharing app too, allows you to set custom urls for shared folders or to create a drop folder url in which anyone can drop files for you. Might be overkill for just files though, i'm pretty much organising/syncing everything on there by now (notes, tasks, kanban, calenders, contacts, etc.)
Also the office extensions are great for collab work but those require even more setup