Is there a secure and good replacement for OneDrive on Linux?
For paid services, there are also native Dropbox client support in MATE, Ubuntu, or Mint desktop file managers etc.
iOS sftp:
https://www.photosync-app.com/home
Android sftp:
https://play.google.com/store/apps/details?id=cz.bukacek.fil...
Practically speaking, I often recommend dual booting from 2 ssd drives for windows and Linux. There are just some commercial software/games that can't run properly within a linux environment (programs like Wine do allow running some Windows native programs, but YMMV.)
https://www.linuxmint.com/ or https://ubuntu.com/download/desktop is a great starter OS.
If you already use a posix system like MacOS, than the workflow will seem more familiar. =3