E2EE should normally solve these kinds of issues, but if the full source is not publicly accessible, and the vendor has the ability to silently push arbitrary updates, it's all only as good as their promise.
Sync is good. Backup is a must. Neither is trivial or easily solved (unless exactly with services like iCloud). Syncthing is great for photos or music libraries, but I dare you to try syncing your source code trees, conflict detection exists but quickly falls apart at the boundaries. Yes git solves that, but you need somewhere to host your code. Tailscale solves remote access to your homelab (which could be as tiny as a RasPi hanging by a cable from the ISP router), but that's again a vendor you must trust. Headscale exists, you can also go for raw Wireguard, but the setup complexity keeps rising, features disappear, and you do need NAT traversal. Oh did I mention ISP spying yet?
So which vendor do you trust with your backups? Good backup solution will offer E2EE; there's rsync.net, tarsnap, etc which are all excellent value for money and are important to consider (vs. homelab hosting) for diversifying geographically (natural disasters are getting more common), and peace of mind (last time a disk failed I just didn't bother replacing it; the next one that fails will have my backups lost). But then again, setup is more involved, and backing up your phone (which always will have very important data on it) is always complicated.
Then of course the OS itself. You can trust your OS vendor or distribution with the builds, or use something like NixOS (let's just say you need to learn a DSL). There's also no reasonable solution for your phone, if you e.g. still want to access your bank.
To what extent are you willing to trust different vendors? How do you assess the risk without deep technical knowledge? How do you implement these solutions without dedicating too much of your free time?