I don't completely disagree with what you wrote. I think you're heading in a decent direction.
However you have to be SUPER careful.
Because if you change something like build speeds for T3, its not just going to affect the super late game, where you're facing T4 assault experimentals.
Its going to affect the T2/T3 transition as well.
And that is one of those things that you really don't want to screw with, because a lot of people believe its in a really good place, myself included.
So fixing this by effectively lowering production costs probably doesn't work, because its just a more complicated version of a build time buff, and that WILL affect T2/T3 balance. It will for example encourage T2 skips. Rush T3 and throw down a bunch of cheap T3 factories, then you can throw all your mass into T3 units. Opponents that stay T2 and eco will likely get steamrolled.
I still feel like the simple solution is a moderate across the board nerf to assault experimentals, or SCU buffs that are so significant that they fix the issue even though you're further increasing the production investment required to get a good T3 formation out.
That second solution is a bit worrisome because of the addition of a quantum gateway to an already expensive production chain... however it does have the side effect of making SCUs a little more viable. Rambos aren't bad, but they aren't exactly commonly used either.
At least with those solutions, you really are only screwing with the very late game.
At the end of the day I don't think I'm alone when I say T3 land went from a pretty good place relative to T4 to to a pretty bad place relative to T4 with the 2018 nerfs. I understand why those nerfs were made and don't have a problem with that, but you have to finish the job. If you nerf T3 to bring it in line with T2 then you also have to nerf T4 to bring it back to where it was with T3.