What's a good setup to back up macOS data and restore to Linux? Background
I have an old MBP which works fine right now.  I have no immediate plans to change it.
But it is getting old, the discrete GPU died, and I expect it could die at any time.  I would like to keep my option open to move to a Linux laptop when this machine dies.  Most of the software I use is either open source or runs on Linux as well and I rely heavily on Terminal/bash already. Considering that and not wanting to be tied to macOS due to not having a solid cross-platform backup, here's my current backup strategy to allow either a Mac or linux based OS to be my next computer for my current data.
modified backups

keep frequent backups, but, rather than relying only on Time Machine, backup /Users/ via rsync as well.  (what file system is my best bet here?)

run preliminary trials on restoring the rsync backups into a test Linux VM.


As you an see - I'm not decided what filesystem I should use to rsync as Time Machine backs up to HFS+ and since I haven't picked a Linux it's not clear if I want / can mount HFS+ to read the backup data.
What setup allows me to restore my data that is backed up with rsync to any Linux?
Use the cloud to store your data.
I actively use three different operating systems:  macOS, Windows, and BSD (I do use Linux from time to time, but generally avoid it).  To access my data on each of these platforms, I use the cloud (a hybrid cloud to be exact).
This can be as simple as having a OneDrive, DropBox, etc. account and a simple USB external drive to back up your data to a highly integrated hybrid cloud consisting of cloud storage, a local NAS with it's own cloud sync clients and external backup.
The point is:  separate your data from your &quot;operations.&quot;  In other words, become platform agnostic.  Because, when you structure your data this way, regardless of what you move to, your data will be ready.
Simple Cloud
If you were to use OneDrive or DropBox, for example, you could store all of your documents to the cloud.  Both have clients for macOS and Linux so both could sync with no issue.
As for backups, both have native backup software (i.e. Time Machine) that allow you to efficiently back up your machine and data to an external USB or NAS device.
Hybrid Cloud
I am a huge fan of Synology NAS devices.  I store my data on cloud services (like OneDrive) and sync it via Synology's cloud sync software back to my Synology.  My Mac (and my Windows and BSD machines) all have external USB drives for backup and my NAS has it's own external drive(s) for backup (I have two).
Is it overkill?  Probably. However, I haven't lost data in over 15 years (I was doing this before cloud storage came out strictly with NAS - nfs, smb, afp, etc.)
Some notes

I use utilities like KeePass's (cross platform password database) data file on the cloud so all of my apps can access it from any device from anywhere

I use local incremental or snapshot backups so I can easily restore data should I type the incorrect rm command.

I keep my data and backups in multiple places - it's in the cloud, it's synced to three machines (where applicable) which are locally backed up, it's synced to a NAS which itself is locally backed up.

I back up my &quot;settings&quot; (i.e. .bash_profile, ./ssh/config, ./ssh/authorizedkeys etc.) the cloud.


TL;DR
Efficient use of cloud services and/or technologies will enable you to store you data in a centralized location making migration from one platform to another quite simple.