Time for some good news so much later !!
As it happens, I've watched for Systemback forks to come along with support for 20.04 and they have, along with added good stuff.
Details of the fork I've used & verified as working well on 20.04.2:
Systemback 2 download | SourceForge.net
Systemback 1.8.8 2021-04-14
Brought to you by: alien-tec
Simple system backup and restore application with extra features for *buntu 20.04
The above is up to v2.0 as of 5/15 & in the files there is 'systemback-master.tar.xz' - which is not useful IMO, so when I got the slightly older version I just grabbed all the DEB files & installed them (6 of them), and the newest files are here:
There is another possibly useful fork, here:
I have not used or tested that one though as the other worked flawlessly for me.