Use syncthing - open source, free, low resource usage, etc.
Most developer/engineer types have a private server and that’s a great place to install Nextcloud. Nextcloud has cross platform clients, supports strong authenticators (U2F), and open protocols like WebDAV. I even use it to sync org-mode files across many devices with great success.
[1] fsync() seems to have been abandoned but still partially works on my phone - only local networks and you have to manually add folders by ID.
https://nextcloud.com/blog/nextcloud-releases-deep-ios-integ...
(And/or you can use any WebDAV client)