I think the comments will prove this is a very polarizing take, and a very polarizing watch. For me, it's by far my least favorite. The bezel reminds me of seiko mods from a decade ago. The fully polished case is a bit lazy looking vs polished & brushed (I apply this same criticism to model DJ too). The Grey dial is interesting for sure, but I'm not really a fan of sunburst and the light blue was just a bit incongruous.
On top of that, I don't know what the YM is. At a glance, it's just a Sub with some details fettled. But with much less w/r. Which would be fine and honestly in-line with an identity as an upmarket sub... IF the YM had been exclusively precious metal and the sub was only SS (or TT at best). But the solid gold Subs lived on, and so did SS YM. And now we have a titanium YM that really feels like it should be called a Submariner, not a YM. Such a weird line with no core logic underlying its existence.
I know that''s nitpicking, but it's nice to have for a watch to have or have had an actual reason for existence that explains why it is how it is...
Wow. Replace nothing. Polish nothing. I know the initial reaction to a vintage watch can be to "restore" it, but restoration is actually generally a destructive process that involved replacing original parts, polishing (which subtracts metal and changes the original contours) the metal components etc.
You have a genuine family heirloom with signs of your grandfathers years of use on it. I'd treasure that wear and tear and only seek to preserve it. You could polish the crystal and have the movement serviced and Im sure it would be well on it's way to giving your family decades of continued use.
Be very selective in who you let service this watch, only allow an expert in these particular references to service it. Might help to reach out to some quality vintage dealers to get a watch maker reference. I might message someone like Eric Wind or Kevin O'Dell for a recommendation.
Brilliant watch. Truly