Format USB drive as macOS Extended (Journaled) and then just copy the files from the NAS SMB share to a local drive and the Recovery mode restore pane will discover it. It does not even had to be restarted.
The copying itself can be done e.g. using the following command from the Terminal (available in the Utilities in the Recovery mode), although any other method will likely work (e.g. using Finder or rsync or such), however in my case I was limited by the fact that the only Apple machine in the household was broken and need to be restored, so I had to use whatever the Recovery mode had to offer. In this case it assumes the SSH public key authentication was set up prior to that, however one can also use password based authentication, if one remembers the password to the NAS, that is):
ssh -o StrictHostKeyChecking=no -i id_rsa vladak@nas \
"cd /mnt/uloziste/macbackup/mini/vladak/; tar cf - ." | \
tar -C /Volumes/T7 -xvf -
The command runs tar to create a data stream (not necessarily compressing) from the files on the NAS and the SSH client will pass that stream to a local tar instance which will create the files.
The StrictHostKeyChecking=no SSH option was used because root‘s home directory in Recovery mode is not writable, so the SSH client would ask for key fingerprint verification on rekeying which happens for longer data transfers such as this one and it would require user interaction to repeatedly reconfirm the key.
The /Volumes/T7 is where a USB SSD drive was mounted to, formatted using MacOS Extended (Journaled).
While investigating the Time machine disk format, I came across a really nice source on https://tommyang.github.io/pondini.org/TM/Works3.html
The copying process above will spend most of the time transferring all the bands files.