Other than GOG's withering "movies" section, I only remember of two that aren't overly niche, DLsite and Fakku (both mainly porn stores). Maybe Itunes' videos are DRM free, but I haven't tested and still it would break the "no app" condition since it's required for payment and download. Also maybe Itchio and Gumroad have something on videos too, since they don't limit the types of media allowed there, but I have yet to confirm.
Unfamiliar with it, but in the regard of instances going down, specially after my previous main instance died with no signs of returning (again), if you find any interesting posts even on instances seemingly stable, I think it is a good idea to back up those pages. Personally, I would propose methods like Internet Archive's Wayback Machine and its alternatives like Archive Today, the print to PDF option from browsers, and/or saving a given page as a MHTML bundle.