NVidia released a new driver today, and it seems to have fixed the issue. I’m still testing but Thunderbird and Avast Mail Shield settings no longer increase the power consumption of the graphics card.
This is probably what caused the problem:
Bug fix:
GPU idling voltage has increased. [1904229]
Still strange how it didn’t happen at boot, but rather when toggling mail shield. But I’m just glad that this appears to be fixed now 
I’ve updated the topic title to reflect the solution.