Are iPhone DCIM 1xxAPPLE folders chronological?
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.