I’ve read that altitudes can affect performance - air being too thin and performance will lag. I don’t think 3500 ft is considered high altitude.
So when you began your return trip, you had that 1200 rpm idle?
Any shakes or roughness now at 750 rpm?
Take a look for obvious vacuum leaks then
I’d try and tweak the idle back to recommended 950 at operating temp. and see how it goes before any dabbling with mixture setting.
Then it will be on to the other CIS factor’s.
Read this link:
https://cis911primer.com/pages/test_troubleshoot.html