I'm another who had transfailure warning and limp mode as first signs of failing batteries and alternators.
Retrospectively, I knew how long my alternator was not working right based on progressively poor shifting.
I was able to pre-diagnose a subsequent alternator failure based on the trans performance
Rule that out, those are both much simpler fixes than the trans.
If you get the battery load tested on the car, don't do it at the jump points. Do it at the battery. The parts store testers seem to be tricked by the cable run.