Without you've got all the test gear that's appropriate for these things I've found diagnosis more like guesswork. Last year my 135 developed a starting & top end intermittent fault. Did all the usual stuff cleaning carbs,new plugs. All sparking, plenty of fuel etc Took it up to Clyde Outboards turned out to need a new switch pack (£160) it has two. Had the carbs stripped and ultrasonically cleaned.
Now runs like a sewing machine !!
The point I'm trying to get across is sometimes it pays to get a professional in if all the obvious stuff comes up blank. A decent marine mechanic is worth every penny but they're not always the least expensive or the easiest to find.