Razor Socket to GFCI Conversion Kit.. Load terminals vs Pigtail
I'd go with your &quot;Pigtailed&quot; plan
Since everything in this box should be on the same circuit based on the existing diagram having the switched-hot and always-hot sharing the same neutral in the outgoing /3 cable and on the fact that there's only one neutral bundle in the box, we can safely assume that neutral bundle can be pigtailed into to get neutral for our GFCI.  To go with that, we pigtail off the always-hot as in your appropriately labeled &quot;pigtailed GFCI&quot; plan, as we don't know what else that always-hot feeds, and putting lighting on a bathroom GFCI has the downside that a GFCI trip will plunge you into the dark, which is bad news if you're holding a curling iron, for instance.