I’m not deleting them. They’re uploaded to the cloud at the time of creation. I also move them off my phone to my computer every 6 months or so - I do this just in case the upload to the cloud has ever silently failed. I deduplicate the images, so I don’t have multiple versions of the same image with different file names.
For me it’s not entirely about space. I rarely let the device get more than 2/3rds full. It’s also about speed. If I want to pull a photo/video off my phone, it seems sluggish when there are thousandths of files in that one directory.