With the motor mostly not leaking and everything fairly clean, it seems
like I have a trans leak at the gasket for the rear cover. And it only seems
to happen when running. 6 months of sitting and no fluid, but drive for 10
miles and I get this.
Click to view attachment
Is this common? Is it just a matter of buying that gasket and removing/
replacing?

Trans leaks are not uncommon, especially with pinholes. Just drill it, tap it, and plug it, then fill it with oil.

Looks to me that it's leaking fluid from a higher spot?
Just pull it out and reseal it if need be. It's not that bad of a job but you will need to do both sides of the intermediate plate. I would do all the seals while you are at it cause you know it will leak elsewhere afterwards.