I am not an expert only an end user of Graphene, but I would surmise that both a factory reset and a full installation achieve the same goal. After installing GrapheneOS I installed the updates as they came out and was hesitant to factory reset my phone, due to not wanting to break what I considered a working asset. However eventually I factory reset the device and the phone just worked as normal. Now after EVERY GrapheneOS update, I purposely factory reset the phone. I have the luxury of just one additional app to install, then I am good to go, takes 20 -30 minutes at most. I don't think I would do a full install on this phone again, I personally don't think its necessary, but hey, what do I know...