New update, I went ahead and updated another similar RPI4 and afterwards even though everything else was running like before, Duplicati would get into this job loop of completing but crashing the service and therefore restarting the same backup over and over again.
I then remembered I had also tried using the latest build direct from Mono, and after that and rebooting, this time the backup are now working, although the service continues to report the message from the second post. So I did the same i.e. just updating the OS because I had already tried the later Mono, and now it too seems to be fine.
So bascially, I needed to use a more recent Mono v6.12 than the default v6.8 that comes with 32bit RPiOS Bullseye.