That seems weird. I have also had that, and patience, plus using Terminal to try to understand what is happening helpd
The command
printf '\e[3J' && log show --predicate 'subsystem == "com.apple.TimeMachine"' --info --last 24h | grep -F 'eMac' | grep -Fv 'etat' | awk -F']' '{print substr($0,1,19), $NF}'
issued in Terminal shows you the past 24 hours.
What I found with mine is that any interruption gave it indigestion, but that it finished, eventually, after I let it back up a stupendous amount, double my used internal storage plus an external drive with my Photos library