Since vanilla has tree-sized mushrooms, and yet are not touched by your mod, how feasible is it to pull this off ever?

It's being considered with low priority since how the mushroom would grow in steps is unclear.

You could always separate the small mushrooms from the giant ones entirely, although this might force it slightly out of your scope. Instead, you'd have to use a potion on a small mushroom as opposed to bonemeal to even get a giant mushroom, aside from natural gen.

As for stages... https://www.youtube.com/watch?v=us79TMh5Dkk

Mix those up with some more fantastic takes on them as well. Since let's be clear, the only fungus that probably would've hit the same sizes as the ones in MC is long extinct.

There was a bit of discussion regarding this on Mangoose's discord channel. (https://discord.gg/dse4X5V)

At least one image for a possible concept was made.

https://cdn.discordapp.com/attachments/420010799545712643/466784813228097566/unknown.png

well, you could use the way irl mushrooms grow as a base (insert shrug)

I'm no longer interested in doing mushrooms as part of the DynamicTrees base. If I ever do add dynamic mushrooms it will be as an Add-On.