How to format a Time Machine SSD so it can also host the backup for external booting
I ended up verifying this scenario myself, you can restore a Time Machine backup from an SSD partition, onto another APFS partition that's on the same SSD drive
You need to enable booting from external drives on T2 Macbook's, but it's a simple step that you are guided into (Everything available on MacOS Recovery with CMD+R) (You need the root password of the Macbook for Time Machine restore capability and changing the startup security, so if you are going to borrow an actively used device, you either need to get the password too, or do these steps with the owner after they enter the password themselves)
If you try this yourself, the only side effect is the startup disk getting changed to the external drive you just restored a backup onto, the restored OS is regularly usable, the disk benchmarks were 900mb/s read/write when booted from itself (everything kept on working as it was too, the only issue I saw was missing Chrome thumbnails, but Chrome password system was accessible as before)
Overall I think this approach is very practical, obviously you need a secondary time machine since it's risky to use the same ssd that's housing your time machine in case of a secondary failure - It took 35 minutes to restore my ~200GB backup with a 10Gbps nvme enclosure onto itself
Some edge information that's relevant at the moment: Avoid 40Gbps Chinese enclosures, major example is Orico (avoid them like your life depends on it, recently restored a system from an HDD, I had it connected with one of their USB-C/USB-A cables, it turns out the read speeds were 8mb/s because of this faulty cable, while it's 100mb/s if connected with a different cable), I believe they are all manufactured to fail, the enclosures heat SSD drives on idle and have various other issues. I found the Asus ROG Strix Arion 10Gbps drive to be very reliant (seemingly). SSD's don't heat as much (nowhere near 90C, more like 40C) and they are cool when idle. I've tested a Samsung 970 Evo Plus, it was unusable with the 40Gbps thrash enclosure, usable with 10Gbps one, but I wouldn't use it in this scenario, the heat is still concerning. Instead, a WD SN550 Dramless Nvme SSD was both cheaper, cooler and suprisingly faster on average. Since it doesn't throttle due to overheating.