I tried another fix and it worked!
Google / Google settings / Instant apps / Turn off Instant apps
No more "IMS Services has stopped" messages, no more lags, no more failing to send messages!!!
Instant apps are apps that you can open before requiring installation. I wasn't aware that I had any instant apps on my phone.
My experience at the T-Mobile store reminded me to never visit the T-Mobile store for help with a phone problem. Inevitably I am advised to buy a new phone.
Thanks for sharing what worked. "Instant apps" - never would've guessed that was the culprit! I also appreciate your feedback on troubleshooting in the store. While it shouldn't be like this, we are working to improve in-store experiences which include training our store reps on tech support.
IMS service message stopped after going to:
click reset settings
click reset network settings