Hi Matt, don't forget your cam fell to bits and sent metal all round the engine, if you've got low oil pressure at idle when hot I would be looking at your shells mate.
Increasing the spring pressure could just be covering up a more serious problem.

Trying a mechanical gauge is a good idea, also check pressure at 3000rpm after a good 30 mins drive, should be about 45psi.

Does the oil light come on at idle too?