Maintaining the Status Quo in a manageable way
I think your options boil down to either buying more space (even a very usable 50GB only costs me 79p a month) for longer term flexibility or managing your photos thru your macOS iCloud Photos Library to reduce your cloud usage.
This can be done by exporting originals as you are currently doing, or by the use of a 3rd party utility such as PowerPhotos to export to a non-iCloud Photos library on your Mac. You can then delete them from the macOS iCloud Photos library and the deletes will propagate up to iCloud. Or as nohillside originally pointed out, they can be deleted through the iCloud Photos website. In both cases, make sure that Recently Deleted is cleared out once you're sure.
And of course, as has just been pointed out, iCloud storage should not be treated as a backup, so a reliable Time Machine backup of your Mac will also be a good idea.
As for the phone restoration issue, a new phone can be restored over a local Bluetooth connection from and old one. See https://support.apple.com/en-us/HT210216
Severing phone and iCloud photos
If you make sure you have all the iCloud photos synced to the phone before switching off
Settings | Photos | iCloud Photos, then it will not prompt you re. downloading when you do, and all the photos remain on the phone. Similarly if you do the same in macOS Photos, then the photos remain on the Mac.
What it's actually doing, even though it isn't particularly clear, is asking you how attached you are to the photos on your phone. If you want to keep them, then it knows that low-quality space savers may be present on the phone, and it is ensuring that the originals get downloaded to the phone.. Or if you aren't bothered, it asks permission to get rid completely.
Managing photos in iCloud is completely separate to that above. To remove the photos from iCloud but leave on your phone:
On the Mac:
Photos | Preferences | iCloud | Download Originals to this Mac (Tick). Monitor this to completion in the Photos app, at the bottom of the
On the iPhone:
Settings | Photos | Download and Keep Originals (Tick) Monitor this to completion in the Photos app at the bottom of
On the Mac, duplicate the ~/Photos/Photos Library.photoslibrary (or however your iCloud enabled library is named). On an APFS system this will take seconds.
Verify the copied library is not iCloud enabled by opening it and checking
Photos | Preferences | iCloud states "iCloud features for Photos are only available in the System Photo Library"
Run a Time Machine backup to ensure both versions are safe
Disable iCloud Photos on iPhone:
Settings | Photos | iCloud Photos (Off). All photos will remain on the phone. If all photos are not currently on the phone then that is what causes it to prompt.
Then delete the photos through macOS Photos using the iCloud enabled library, or via the website. They will be removed from iCloud, but remain on your phone.
Future photos taken on the phone will not end up in iCloud, so if you want to transfer them to your Mac you will have to import to macOS Photos, or use the Image Capture utility, both with the phone connected via USB.