To conclude this thread… tl;dr: IT WAS THE VALLEY PLATE LEAK!!!
Went back to shop3, asked them to pull the intake manifold to inspect/advise per the endoscope pictures. They initially just pressure tested again and said “no problems found”.
Sent and reviewed the images with them and we agreed...
Aha. I think I found something! Since having the block and radiator tested, I’ve been monitoring the coolant. It’s been mostly stable, slowly dropped the last couple of days.
Got a small endoscope camera to poke around more. Today, going in thru the front of the engine, there appears to be a...
Looking for suggestions on next steps, things to pay attention to, etc
2013 LC 104k
Got the truck in May 2018, no issues/problems observed
June 2021: replaced the radiator due to the beginnings of the radiator crack at "shop1"
No issues/problems observed at that time.
No overheating etc...