I'm not sure about the Windows specifics, but I can relate some experiences with the iCloud option and adding older unmanaged photos in recently.

The iPhone in Collections view always respects the date and time stamp in the photo metadata (exif) for sorting. The only place it ever look at a different date and time stamp is in the Albums - All Photos view. This defaults to date imported. Albums allow custom ordering but I believe they default to the exif data as well.

Any photo you copy off and work with the file should go right back into your collection when you upgrade to the newer phone. The details of that on Windows I don't know though. My guess is you would want to use iTunes to sync them back instead of trying to copy them back in the DCIM structure.

iCloud Photo Library I find is worth it for my needs. The phone will not have to keep the files locally, all originals go into the cloud storage side. On each Apple device you get a setting to keep originals locally, or replace them with space optimized files tailored for the device and it's free storage. This would free up space on the phone, and would rely on it's internet connection to pull down the original photos if needed (such as exporting them on the iPhone to another app). The downside for your use though is there is no way to access iCloud Photo Library with a Windows native app, all access there would be via the icloud.com web site.