Maybe nextcloud? IDK I just use Borg. But Nextcloud allows that type of syncing that you describe, I think. I run a small nextcloud server for other purposes and don't use that feature.

https://gitlab.com/GwendalJ/package-transporter

Not useful yet, but it implements a per-app checklist.

iCloud shit is only 'convenient and beginner friendly' because you're paying someone else to do all the work and maintenance for you.

Every system has its own processes. If you want Apple software and services use Apple. If you want Linux use Linux. Do not expect either to be like the other especilly at such a micro level.

As far as Linux and beginner friendly, buy a device with Linux preinstalled just like you do with Apple. As far as user setting and apps. Get a notebook and write them down, and avoid deep customizations. As far as backup get 3 USB drives and backup your home directory with rsync or one of the other solutions. As far as restore, have install media and just reinstall from scratch then layer in your configs and apps and then restore your home directory files. For file sync and app sync functions, Nextcloud is helpful and you can pay for a commercial host, set it up yourself, or use a product like Synology. You frankly could use Dropbox, Proton Drive, or one of the others also. But think carefully what is actually needed. Cloud stuff is heavily promoted by the big providers presumably for lockin reasons and to mine your data but it is not really needed for most things. Get to know your distros builtin emergency startup tools and have a live distro like the live install media available and know how to use them.

Linux is about options but for simple beginner like processes it is best to stick to the basics.

As others suggested the backend is probably already installed on most computers but not setup, namely :

• ssh to manage passworldless across multiple computers (you need that for data to be safe)
• scp/rsync/rdiff-backup to actually copy the data thanks to ssh keys

One could imagine a dedicated user per machine that is for read-only of data (maybe after some encryption, limited to very specific directoriess) and another for storing only of data (with no access except to write on disk and with a maximum quota).

What this highlight though is that the centralized managed cloud model is challenging to replicate as purely p2p at home, namely backing up your phone to your desktop might be find but the other way around, probably not. Maybe even more challenging, what do you actually backup? I would argue your home directory but... clearly not your e.g. Steam games (humongous) or other backups or video files downloaded from the Web. So... probably a select set of directories in home then, but which ones? ~/Documents only? This specific part implies some decision from the end user.

Anyway I believe all the tools are there, but I think what most people lack is to view the result and for that maybe some equivalent of https://gitlab.com/ikus-soft/rdiffweb/ which shows when was the last backup done, how big it was, etc basically some form of visual to feel safe.

Finally to skip the CLI key management part the closest I know, for end users, is KDE Connect https://kdeconnect.kde.org/ which I discovered after building my own https://git.benetou.fr/utopiah/offline-octopus kind of equivalent, namely a way to use devices on LAN. Backup is not a default feature though but could be.

For backup, one can simply rsync their entire home directory.

NFS for shared directory, maybe syncthing to have locally synced stuff, scp for sharing files?

you want a solution for your computer? or for your phone?

if it's for your phone and it is still iOS then no, the Apple Ecosystem is closed, there are some reverse engineered offers but they tend to be patched out or not be reliable.

Linux Mint + Filen