I had a similar situation on a tractor I was working on..
New pumping plungers in pump, I overhauled it so I know it was put together correctly..
I cranked and cranked on that thing till the cows came home.!!
I loosened the inlet line and fuel would POUR OUT..
I did a compression test.. it was low but acceptable..
O/h the injectors..
Same thing, crank and crank..
As a last resort, I blew air FROM THE PUMP LINE BACK THRU THE FILTER AND INTO THE TANK..
“I knew” that wasn’t the problem because I had “plenty” of fuel at the inlet when I was bleeding it..
All of a sudden I hear a POP on my first blast of air..!!
“Something” was in the line restricting the the fuel flow just enough to keep the pump from pumping..
I thought I was getting enough fuel, but after that POP..I definitely was getting enough fuel.!!!
I turned the key and she roared to life..
I don’t recommend u start at the pump inlet seeing you have a supply pump and could rupture the diaphragm.. but I would pull the feed line off the supply pump, LOOSEN OR TAKE OFF the tank cap and give it a shot of compressed air.. and listen for bubbles..
Good luck