How do I factory reset my "unavailable" iPhone?

Space is full and an old update on your iPhone shouldn't be a problem with erasing your iPhone, but a deleted Apple Account will. To fully erase an iPhone back to factory settings, Activation Lock will need to be removed. The easiest way is by providing the Apple Account credentials, but if you don't remember it and it's deleted, you'll need to proceed with the next option, which is submitting a claim to Apple with the original proof of purchase receipt.

How to remove Activation Lock - Apple Support

If it is signed into an Apple account that no longer exists and find my is enabled, you're going to have to go to Apple with the original proof of purchase from Apple or an authorized retailer to have the activation lock removed.

If it is activation locked and you can't provide adequate documentation, it's a brick.

Tim I think you hit the nail on the head.