Hi,
I have to agree with you. Things are changing. Over 41 years in the business, many things have changed and have presented new challenges. However, I never (in 41 years) heard of a transmission issue being caused by spark plugs that are not causing a misfire. Actually, even if they were causing a misfire, I don't see how it would cause this.
Based on the description and symptoms, I suspect it has something to do with transmission line pressure. In the transmission, there is a transmission fluid pressure switch, pressure solenoid, and several other electrical components.
Not being there does make it a bit more difficult, but based on the delayed engagement, the first thing I would check would be line pressure. If it is low, it would cause delayed engagement and delayed shifts.
If you want to see what's involved, I attached the diagnostics below for your review.
Let me know what I can do to help.
Take care and Happy New Year,
Joe
See pics below.
Images (Click to make bigger)
Friday, December 31st, 2021 AT 8:59 PM