You’re asking if it could be TCU or possibly transmission but HAVE you tried a different TCU?

$80 bucks on EBay and you’ll know or can rule it out within an hour of installation. Appears to Be an easy option.
you appear to be pretty deep into This already. What’s another $80 bucks just for peace of mind.

just my thoughts and it’s less than a night out with drinks. I hope you find a cost effective solution.