How many times in that 6 weeks have you tried to restart it (eg by logging out and in)? Because every time you do this, it restarts the sync process from the beginning. With 22,000+ times it will take some days to complete even if you leave the mac on 24/7 connected to power and with sleep disabled.
I’m guessing this started when you upgraded to Monterey>
(For reference, my 5000 item library took 48 hours - so you can expect in the region of 8+ days - if you leave it on 24/7 as described.
One other thing worth checking - do you have an “unable to upload folder” or if you create a smart album with the condition “photo - is unable to upload to iCloud” Does it show anything?