I too ran into this same issue when trying to upgrade my iPhone 13 to 16 Pro (iPhone 13 was/is running iOS 17.7.1 without issue). I ultimately had to revert back to my iPhone 13 due to the same exact "calendar/battery drain/iPhone over heating" issues.
I think I'm reading this post too late (already reverted back to my iPhone 13) as it never occurred to me that my iWatch could be causing this problem, but looking back it makes sense.
When I did the iPhone13-to-iPhone16 "data/settings" transfer I forgot to un-pair my iWatch (from my iPhone 13) prior (the thought never crossed my mind), and I do remember needing to re-pair the iWatch to my new iPhone 16 Pro.
I'm now wondering if this is somehow the root cause (not un-pairing my iWatch from my old iPhone 13 before performing the "data/settings" transfer to my new iPhone 16 Pro).
Makes me want to try it again. I'll keep you posted.
@edzfive - did this resolve your issue?