Yeah, I would not call those “dropouts”. That term is usually associated with a total loss of data for a period of time.
Those are notable low values/spike, but I am not sure what is driving them. Sounds like you’ve basically covered the battery, although using the better type might help.
I do think it’s worth a test on ANT+ if you have that option. Seems BLE is most recommended, but I know some people still have better luck on ANT+ with their situation and noise in the local environment.