I thought (also most people said) keeping the Backup in same partition as root defeats the purpose of Backup and brings certain inconvenience like can’t just delete the partition. Also I don’t know if its possible to restore a backup from a partition to the same partition itself.
Use BTRFS if you can.
I’m aware that BTRFS has certain adavantages. But the whole BTRFS is alien to me, as I’m new to Linux. Also I assumed that BTRFS doesn’t have enough community support as ext4 is default on Linux and many people just aren’t bothered to change it.
If you can, ditch dual boot. If the reason of keeping Windows is MS Office or Adobe apps, you can install them on Wine.
I only use Windows for DaVinci Resolve Free. And for the possibility of requiring Windows exclusive programs in the future as I’m an Engineering Student.
System wide flatpaks are in /var/lib/flatpak/app. Flatpak installed for one user only are installed somewhere in ~/.var. Keep in mind that home directory is not backed up by default.