I just use syncthing and set up the appropriate send /receive permissions for each folder
When I want to access those files remotely, I just sftp / ssh into the server. (Someone more knowledgeble than me can help you with that part, I just install Tailscale on my devices for remote ssh)
sure it may not be elegant, but is pretty easy to maintain in the long run (see complaints about updating / setting up nextcloud in this thread)
maybe this will help with setup. Note he is doing bidirectional sync, but one way sync works too:
https://www.youtube.com/watch?v=dBVTedUWbfg
edit: forgot to mention, this is cross platform across Mac, Windows, Android, and Linux. Not sure about iOS