I think we can agree your engine is showing signs of internal wear. There are a few things you can try to minimize the symptoms however:
Fuel/Air mixture
Try adjusting the mix to a smooth idle with the adjustment screw as far out as possible. In other words, adjust to the leanest setting that delivers a smooth idle.
Timing
Set the timing precisely. Power timing tends to err on the advanced side which can increase blowby. You report that you have the tools to find Top Dead Center accurately but you should create your own timing mark for timing light use. Thatch on this forum is passionate about timing and has come up with a couple of timing mark methods that are much more accurate and easier to use than the crankshaft roll pin. If you have a sheet metal flywheel cover, here's a picture of one of his methods. With the window and notch cut in the cover, mark the flywheel with a white paint line so the marks align at #1 Top Dead Center. With these marks, set the timing at idle (assuring no advance influence) with a timing light. Moyer has a timing handle (different from the hand starting crank) where the handle is aligned with the crankshaft roll pin. It moves the crankshaft both directions.
https://moyermarine.com/product/igni...misc_01-1_285/
Another method is an accessory drive pointer. It is fabricated from 2 hose clamps. I have this and it works well.
Addition of top oil in the cylinders
Unless you've had a water incursion or are dealing with sticky valves, I'd lay off the Marvel Mystery Oil treatments for now. It is certainly contributing to your exhaust smoke. It would be interesting to see how much of the smoke is from the engine and how much is from your oil treatments.