Tester shows Circuit wired correctly but GFCI Trips under smallest load
White neutral is connected to the panel neutral bus (verified not
ground). The black wire from the 10-3 is connected to breaker.

Last I checked, (I don't use GFCI breakers, normally, I just put a GFCI as the first thing in line) the neutral needs to connect to the GFCI breaker neutral connection (and depending on design, you may need to connect a pigtail from the breaker to the neutral buss) otherwise the GFCI cannot possibly work.
A GFCI compares the current on the hot and neutral, and trips the GFCI function on a difference between them (5 mA nominal) - if it's not seeing the neutral return current, it will see an imbalance from the hot it does see and the neutral which it will see as 0mA if the neutral wire is not connected to it correctly.