Is there a way to download the whole album as a backup, not just single photos?

Yes — the file manager has a Download Album action that grabs the lot, and guests on the full-access side have a "Download all Photos" button as well. Before you do anything drastic like deleting an album or downgrading your plan, running a full download is a sensible insurance move. It's also how you'd hand over an entire edited delivery to a client who wants the master files on their own drive.