iCloud mistakes new MacBook Pro (14", 2021) for old MacBook Pro (13", 2015); makes it impossible to sync Safari data or use Airdrop

Hi,

This is subtle and confusing issue. I bought a new 14" MBP to replace my aging 2015 13" MBP back in November. However, since then, iCloud features in Safari have been intermittent to unusable. iCloud tabs do not work, at all. The Reading List syncs intermittently and incompletely. Handoff does not function, either.

The particular way Airdrop fails makes me think that some misbegotten file somewhere in iCloud still references my old Mac which has been wiped and gone off to pasture for months now. When I try to share something, the Airdrop device button says "MacBook Pro" and shows a picture of the old Mac! When I tap it, it goes no where and does nothing. The same happens from the MBP to the phone. (Please do not ask if I have Airdrop turned on, it is on, it has always been on.)

I've tried changing the device name of the new Mac, which does not seem to have triggered the rename uniformly (it shows in Find My, but not in the Share Sheet and AirDrop button). I have tried turning off the Safari toggle in iCloud, waiting, and turning it back on, with no change.

Is there some hidden preference file I can delete from /System/Library to fix this or am I going to need to scorched-earth this problem by systematically signing out of my devices, one by one, to force iCloud into synchronizing anything?