After Tahoe 26.2, CONTACTS and MESSAGES are a mess on my iMac

I notice that my iMessages and Contacts are all OK and up-to-date in my iPhone...but NOT on my iMac. Weirdly, all the messages still show up, but some of my most frequent and longstanding contacts--who all have iPhones, I believe, including my husband--DO NOT SHOW UP IN CONTACTS.

In addition, in iMessage, the messages are there but without identification of the name of the person associated with that phone number. See attached screenshot as example from iMessage. I spent 2 hours on the phone with Apple Support. No improvement. Followed all the steps suggested in a late 2025 thread on a different thread. No improvement.

So I'm starting a new thread. Please HELP! This is driving me batty and wasting so much time.