Use a test light for your voltage checks. Volts mean nothing, the test light will measure/show you have voltage when a load is provided. You can have 12 volts but no amp/load ability. I have found many a problem like this and it’s usually a corroded connector that slows no load voltage through, but once load is placed on circuit you get nothing.
Thanks for the help guys, I think I solved the issue. I swapped in the original fuel pump relay I kept as a spare and it's cranked every time. The faulty relay I removed was OEM and only three years old, not sure if there's an underlying problem. They both passed the FSM test and clicked when connected to 12V so I assumed the newer one was working properly.