First thing, disable all auto backup on your phone. This is step 1 for anything.
If you have access to a computer, log in to Photos through a browser and delete images there and it won't affect your phone's storage. Maybe test it with an image if lesser importance before you bulk delete.
You could also move your locally stored photos to a different, temporary folder. Then delete the backed up ones in Photos. Then move the local files back.
I wouldn't rely on the Google Takeout images. If the standard settings applied, the images will all be in Google's compressed format. Granted, most people in the world couldn't tell the difference. But it might be better to keep the best quality for the future.