I have a 1986 Hunter 28.5 with a Yanmar 2GM20F. What you describe seems to be a somewhat common problem with aging Yanmars and their wiring harnesses. My engine began to develop the same symptoms a few years ago. Perhaps like you, I began the process of trying to find and fix the problem by taking off all the connections from the battery to the starter solenoid and cleaning and tightening them (turned out to be worthwhile as I found some loose crimps, but did not fix the starting problem).

I also replaced the starter button, which also did not fix the problem.

Eventually, I read about, and did the following (which DID fix the problem).

I bought a 12vdc relay at an auto parts store and installed it across the starter solenoid terminals. I added a 30 amp ATO fuse in the hot wire. The whole thing cost about $30 and Ive had no starting problems since.

I personally think that the cause of this commom problem is that as the wiring harness ages, it can no longer deliver the needed power to the starter. But replacing the entire harness would be an expensive and time consuming job. This fix worked for me; perhaps it will for you, too. Good luck as this is an annoying problem.