Apologies for the length but appreciate your help.

2006 Lacrosse CX V6 3.8 w/4T65E transmission, bought Feb 2022, only 61,000 miles. Diag prior to purchase revealed a "#1 solenoid" code. Given the cost to replace solenoids, shop recommended keep driving.
Then I noticed it. First 20 mins of driving around town, no issue. Then, progressively longer delays downshifting from 2nd to first until only 2nd gear starts. Same issue on highway when warm, unable to move into D from 3rd. Turn off the car, let sit for a few moments, get 1st back but the cycle repeats.

Took to a local reputable transmission specialist (in business over 50 years). Indicated the problem, their scanner threw the same code. They dropped the tranny, replaced the solenoids and associated sensors, upgraded some internal OEM parts to better aftermarket parts, installed a TransGo shift kit and put on a new wiring harness. They also said there would be a relearn process. After ~5,000 miles of "relearning", the same problem continued.

Took back to transmission place, they replaced all the solenoids and shift sensors again, put on a new transmission manifold (?) and something else. Drove off but the same problem continued.
Took back again and since most moving parts inside the transmission have been replaced, deduced the problem was electrical. We replaced the PCM (programmed at dealer) and now new codes appeared now pointing to an issue with the throttle body. 3.8 has the electronic fuel injected throttle body w/TPS built in. VERY hard to find located a reman from China. New PCM and throttle body, problem went away on short and long drives - at least I thought it had. Then ~500 miles later, the same problem returned.

Small town, people know each other and not speaking automotive, I had a reputable car mechanic who has a good relationship with my transmission guy to run diag, talk and compare notes. Car mechanic threw out the possibility of a bad torque converter (not replaced in all this to this point) but transmission guy dismissed this, told me his reasoning but again over my head. Stumped, worked on many 4T65E'* and never seen this before, he moved to the transmission valve body. He'* never replaced a valve body on a 4T65E before but thinking there could be hangup of solenoid movement inside the current valve body, we ordered a Sonnax - top of the line as I understand, new solenoids included. Test drove yesterday and the same problem is there.

Can someone please help me with reasonable next steps of items to look at, and in a language I can communicate with my transmission guy? Thank you once again in advance for the help.
Matt

We can't teach you the language by way of forum posts. If the subject matter were that simple, there wouldn't be transmission specialists.

I agree it'* likely not the torque converter. This issue appears to be a control issue, so solenoids and valve body.

Transmission guy should be able to test line pressures to figure out what is incorrect while the issue is occurring, then troubleshoot from there.

Also, the PCM should have codes stored for this severity of issue. What fault codes are stored in the PCM?