Are you willing to try Release: 2.0.4.34 (canary) 2019-11-05 which seems to be doing well so far, except for a problem with “Stop after current file” that’s being worked on (don’t use that)? You can’t easily revert after using this (due to database format changes), but it sounds like your current backup situation is very poor, so maybe you can just give up, start again on the latest, see what happens, and report any issues.
Canary is unpredictable because it’s usually had little run time, but right now it’s nearly Beta quality, and you can avoid more Canary if you like by changing your Settings to Beta channel and awaiting its arrival.
If you’re not willing to pick up the fix in Canary form, a workaround for Beta might be to use –snapshot-policy to use Windows VSS to avoid the confusion caused by trying to backup a file while it’s growing…
Possibly it’s a different problem, but that’s the one that got fixed (but won’t be in a Beta until next Beta).