I think the G&K change of requiring a DoF to sign an RA will be a pretty good balancing force. Maybe they'll still be too strong, but we'll see.
And while I agree that the GP tile improvements are generally too weak in comparison, I think it's hard to balance because while you'll get more hammers/beakers over the course of a game from an manufactory or academy than from a hurry or a bulb (if done early on, at least), the opportunity cost of having to wait so long for a return on investment means that you'll probably get a lot more use out of the instant gratification options. Maybe you'll get, say, 5x the beakers over the course of the game from building an academy than bulbing, but when you can get Rifles 20 turns earlier by bulbing, or save up 3 scientists and get it 60 turns ahead of everyone else, well you just won. Doesn't matter that by modern era you'd have gotten more gross beakers because instead you just killed everyone.
Then again, that doesn't mean the improvements are too weak they're actually very strong, and it would be hard to make them much stronger without breaking them. Making them scale with tech was a step in the right direction, but I think additionally the scientist/engineer one-shot abilities need to be made a little less exploitable. Also, the tradeoff between immediate benefit and long-term benefit is fine to have, but as it is being able to rush a wonder or tech slingshot is just so much better because of how abusable that mechanic is. I'd recommend something like this:
Hurry Production Consumes the Great Engineer, giving the city +50% production (or maybe, say, +10 base hammers) for 10 turns.
You still balance long-term vs. short-term (vs. golden age, and vs. donating the GP in G&K), but you can't guarantee you can get whatever you want as soon as you want it, which is why hurry production is way too good. Same thing with bulbing. Instead of giving you the tech you want, or instead of just making it another method of getting an RA effect, you could have a GS add, say:
Accelerate Research Consumes the Great Scientist, giving the city +100% research rate for 10 turns.
Again, doesn't let you beeline by rushing an expensive tech, or worse, slingshotting, but it does make you choose between planning for immediate benefit vs. long-term gain. Also, that would give smaller, taller empires an added benefit that IMO they could use. At least it gives another knob to tweak. If it seems like tall empires are too good at science but really lacking in hammers, make the engineer a very large city-based bonus and the scientist a smaller but empire-wide effect. End result is wide empires get more benefit from the GS and tall empires with their huge cities get more benefit from the GE. I don't know if there's actually a real imbalance between the two, but that could be a unique way to bring them into line a little. (Not saying tall and wide should function identically, just that if they're too out of whack this could help remedy that.)
I should also clarify that I think landmarks and customs houses are pretty terrible. But so are artists and merchants in general, so that's more of a "GPs need to be given an internal balance pass" than "buff these two tile improvements" IMO.