If your pressure gauge is one of the first things inline after the pump, it can show high pressure with very little flow. If there is a blockage and the oil can't flow enough volume beyond the blockage/restriction then it cause issues.

You cannot build pressure in a system unless there is enough flow to offset the leakage. Normally all balanced oiling systems can produce more volume than needed, the passages and relatively small bearing clearances produce restrictions and allow pressure to build. Restrictions are not bad if you can get enough flow through them, if the restriction is too much then it is bad. It's all about balance, carburetor jets and air bleeds are restrictions, if they aren't too big or too small they make things work.

Pressure pushes things, on engines pressure pushes fuel and oil to where it is needed. That is the function of pressure. Volume is important also, there has to be enough volume so there is a good film of oil between the bearing surfaces. Since it is an open system unlike a tire without holes, the pump has to be capable of supplying enough volume to match what leaks out.

One thing most people do or have done that relates to hydraulic (oil pressure and volume) is use garden hoses. Attach a hose to a waterline (faucet). Put a shut off valve on the end, and then attach another hose. If you turn on the water it will begin flowing through the first valve and first hose, then through the second valve and into the second hose. Now start closing the second valve that is in between the two hoses, and the pressure in the first hose will get higher, and flow into and out of the second hose will drop. You may notice the first hose swell as the pressure builds. A restriction has been created with the valve. If you turn it all the way off flow will completely cease, but there will still be pressure in the first hose.

Anyone that has worked with water systems that branch out to several outlets (watering the lawn, house plumbing, etc.) understands that you need to have enough volume and pressure going into the system to service all of the outlets. If one branch is too large, it starves other branches and if you add a restriction on that branch it will force the flow to the other branches.

An oiling system is essentially a series of passages or galleries (hoses). Restrictions (analogous to un-adjustable valves) in the system could be the size of the passage, and the bearings and lifters are control flow according to the clearances.

Your problem could stem from an oil delivery problem, it could be a clearance problem, it could be your rods weren’t as good as you thought.
There was obviously a problem, or perhaps more than one. The thing is, if it was a problem with the rod, bearing, or crank pin it has been destroyed and you may never know for sure if that was the problem or not, unless you find another smoking gun. What you may need to do is check everything that is left and all new parts very well, reassemble, and go from there.

I don’t know who checked your parts before. They may have been dead on, or they may have been wrong but I will say measuring parts accurately and in a 3 dimensional fashion is not as easy as it might seem and experience and technique are very important.

If a pump is sucking air through the pickup it will still move oil and build pressure, but foamy oil doesn’t work too well. Something to keep in mind. One more thing, was the oiling system primed before the first start up?