define "self custody" ..
In this case I think gold, cos you dont own "physical" bitcoin, just a key to spend certain UTXOs.
Every node runner technically "owns" all bitcoin, just cannot spend most of them.
Discussion
Good point on all UTXOs being available on every node with a limitation on what I can spend
Self Custody to me is not about physical or digital or UTXOs .. it is about - does the real owner hold it under direct control or a third party (human) holds it on my behalf .. if I hold my UTXOs in say a coldcard ..I self custody bitcoin ...