Well, I started a chat in Messages with support, and we went around the block a few times, but at least that guy seemed to understand what I wanted to do. He couldn't get it to work for me, so he passed me on to phone support, and I've been talking for 45 minutes to a woman who refuses to understand what I'm trying to do. She keeps wanting my to make my Memoji my photo ID by changing the photo I have on my card in Contacts. Then she asked the engineers for help, and relayed their response to me, which was this article on animated Memojis: Use Memoji on your iPhone or iPad Pro - Apple Support
As "proof" that I shouldn't expect my Memoji to sync to my Mac, she showed me this list of compatible devices: iPhone and iPad models that support Face ID - Apple Support and clearly the Mac is not listed. Problem is that neither my iPhone nor my iPad is on this list, yet they sync fine. So now I'm on another lengthy hold, but I don't think my chances are good of solving my problem, at least with this rep. I've asked to be passed on to someone who might listen to me long enough to understand the problem I'm trying to solve, but so far I'm just on my third lengthy hold. Very disappointing.