F16GUY
I’m military through and through and if I had to divert for fuel (I have a couple of times) I would declare a PAN as a matter of course.
The reason is that, upon reaching my diversion airfield, I would then be out of options and would need landing priority. This assumes that at the start of my diversion I was at my minima (I was when I did it for real).
When you reach your planned alternate you do not carry enough fuel for another alternate so it is your last chance saloon.
The question of PAN vs MAYDAY may carry a personal (or company SOP) element to it. In my mind (without giving it too much thought) it would depend on weather and traffic. If it is a VFR transit to the alternate and light traffic a PAN would suffice. If the situation was making me twitch then a MAYDAY may be required.
BV