I can’t imagine it having any oil pressure to speak of if a plug were missing.
And one would need to be really loose........ like ready to fall out before I think it would have that much affect on the pressure.

If you’re still employing the factory style through-the-cam top end oiling, I don’t see how the oiling to the top end could be the problem.

To me, that leaves a restricted or loose oil pump pick up, a restriction coming out of the pump into the block, a restriction in the pass side oil gallery........ or excessive bearing clearances somewhere.

Hopefully priming with the intake off will reveal something.

I just had a 360 on the dyno the other day.
It has edm lifters, pushrods with holes on both ends, and rockers than can be used as pushrod oiled.
It also retains the factory through-the-cam oiling(double oiling to the top)......... and plenty of side clearance between the rocker pairs.

Stock replacement type pan....... after several back to back pulls the oil pressure was still 30psi@1000 rpm.
About 65psi@6500 with Driven BR 15/50.