One alternative might be to host your own Nextcloud Server. (There are providers that will host them for you for under 10 € a month.) This solution might be a bit overkill, but you would get a fancy cloud storage solution that you can use as an alternative for other services as well.

Another option might be "Magic Wormhole". But this might be a bit techy and not as user friendly as other solutions. But it can be fun (if you are a but techy) is basically peer-to-peer. https://magic-wormhole.readthedocs.io/en/latest/

Another alternative to look into might be "Send". https://alternativeto.net/software/send/about/