Yes I agree with you but personally, I'm not confortable with having one single backup of my seed, even if I would carve it in stone.
Completely agree. Once back up is no back up at all, especially when most people store their only back up in the same location as their computer or hardware wallet (i.e. at home). Two back ups in two separate geographical locations should be the minimum.
Running a full node is not an acceptable solution for a random user, and Bitcoin is supposed to be safely usable by everyone.
Sure, but here we are talking about privacy focused users, not random users. If you are privacy focused, then there is no escaping that you must run your own node or you will be leaking data to a third party.