you might also try mkusb-minp as it works with
debian live distros. I've been using it with Trixie weekly builds
e.g.
sudo -s
./mkusb-minp -p {debian-live-iso} /dev/{usb}
So, this will install a persistent system on the usb. You can boot it, update it, configure, maintain, etc. Except for the kernel.
When a new kernel is issued you can mount partition 3 of the usb, the persistent partition (the other partitions are essentially immutable) and do an rsync backup (e.g.):
rsync -axHAWXS --numeric-ids --info=progress2 {mounted partition 3}/ {backup-area}
Note the appended slash is necessary for rsync. Now unmount and
install the new kernel:
umount {mounted partition 3}
./mkusb-minp -p {debian-new-live-iso} /dev/{usb}
mount {usb partition 3}
rsync -axHAWXS --numeric-ids --info=progress2 {backup-area}/ {mounted partition 3}
… the state is maintained and the kernel is updated