I'm not sure if it is the parameters under the dataset that needs to be different...
Did Catalina ever back up correctly to your previous versions of FreeNAS or TrueNAS? I'm new to TrueNAS-12.0-U1. I was on the 11.3 train for a long time (recently migrating from FreeNAS-11.3-U4.1).
A long time ago I went through Time Machine hell (constantly having to recreate the backup). I have been on Catalina for a while, and it seems to work okay with the settings I mentioned for 12.0-U1, and also with my previous 11.3-U4.1. But I've only done a handful of backups on 12.0-U1 so it is too soon to tell.