What is you compression? Have you tested the stator or anything else? Stop revving it in neutral before you wreck it. Does spark jump a 7/16" air gap. Is your fuel fresh? Any water in it? Oil injection checked to be working?
The trim sending unit is usually a plastic switch that goes up and down with the motor. A continuity test will help you test that. It could also be your gauge.
I did not say they were good but there are 2 different versions. The design 1 has a bushing and design 2 has bearings. Mercury sells that lower unit brand new for around a $1000. I am also pretty sure that the older bushing can be replaced with the newer bearings.