Are iPhone DCIM 1xxAPPLE folders chronological? For example, am I guaranteed once 100APPLE is full, it will never change?

I am manually backing up the folders and this would make my life easier. Also when moving to a new iPhone, it means I can continue to only back up the new folders.
No, I don’t think you can count on that being guaranteed. 

Yes, chronological filling first is typical and unless someone messes with the storage or counters / settings you’re pretty save with that assumption/generalization. 

For my sanity, I grab all the images and process later if I feel I can’t just store all the images from each device. So far, my main storage grows faster than the size and accumulated count of images since iPhone launch 10 years ago.