[edg5000]

For files I use the open source Material Files, which supports SFTP servers. So I just have a little file server. For calendar, because Google doesn't reliably support background services, it's best to use a calendar app with builtin caldav sync. For carddav, I use a background sync app though (davx). Super lame that this is not built into android, not even into lineage. You'd think someone would implement native caldav/carddav sync? Maybe this is my calling haha.

[noisy_boy]

I'm telling that someone who comes up with a decent file sync setup between iPhone, Android and Linux/Windows without charging a monthly fee will make some good money on one-time buy fees alone. Dropbox etc can do things like these but I'm not interested in paying monthly fees for using my own storage across devices.

[edg5000]

For my desktop systems I do a nighly rsync to a central onsite server, which does nighly encrypted ZFS send to an offsite archive. I suppose a script could trigger on file change using Linux file watch features (exposed via CLI and C headers). What I'm not sure about is wether two way sync with rsync is possible. This could run as a simple daemonized bash script. On error the script would just retry. Rsync would have to handle conflicts, it probably has features for it. Paste this comment into a coding CLI and boom, you have your solution :). Does need rooted Android for running shell scripts on boot. But a good coding CLI can log into your phone and set everything up.

[fragmede]

Have you tried KDEConnect?

[noisy_boy]

Every day, its great. But it is more suitable for adhoc sharing, not keeping pairs of folders in sync like Syncthing does.