Liv - I can't help you with the Wagyu question specifically, but my grass fed Dexters sure take a long time to get to a slaughter weight. 30 months seems to be the minimum ... and I'm thinking 36 or so is probably better.
I can't address the question about the quality of the meat with time ... although there does seem to be some correlation to flavor and diet (and thus time of year).
Not a Wagyu owner, but did grass feed and finish highlands and commercial cross steers. Heritage breeds take longer to finish than commercial breeds on any feed and ALL breeds take longer to finish on grass, they don't develop fat marbling until mature. Grain fed cheats that by making obese teenagers, just like kids today on the standard American diet.
"You must be the change you want to see in the world." "First they ignore you, then they laugh at you, then they fight you, then you win." --Mahatma Gandhi
"Preach the Gospel always, and if necessary, use words." --Francis of Assisi.
"Family farms work when the whole family works the farm." -- Adam Klaus
I run both 100% Wagyu and Wagyu/Angus cross. I would recommend holding wagyu longer, and 3 year is better than 2....but 2 years is still excellent! Don't worry about getting to perfection - harvest the meat when the time is right.
For any grassfed beef, I would wait until June or so to butcher in our area, since the winter body is more in survival mode than storing up fat.
I manage my pastures to get a lot of summer hay and I rotate on the 4 highest fields on my river bottomland (but there isn't much green grass) while feeding about half a bale per full size cow per day. I have the cows together with their babies from this year, and most of the yearlings are in a separate field with the same hay and thin grass.
In our wet area, most of the hay put up is overmatured so calves will gain some winter weight but cows will lose some.
We couldn't get nearly as many animals through without the winter hay - it would probably be one-third to make it through