I think the ultimate goal should be for SCUs to be used as support units in small numbers in the mid-late T3 stage. I don't think there should ever be a transition to "full" SCU production over T3 units.
If you make Quantum Gateways assistable, the Quantum Gateway in effect becomes an HQ, because you pay a lot for the initial building but then subsequent BP from engies is much cheaper. That would make scaling BP into SCUs much easier (and allow for bigger transitions into SCUs).
The idea behind making Quantum Gateways unassistable (whether by increasing the BP/BT significantly or just straight up making it unassistable) is that SCU production doesn't scale. Unlike land/air/navy production, where doubling your BP at a certain tech doesn't cost 2x as much (e.g., you've transitioned to T3 land - the Land HQ costs 5220 mass for 90 BP, but your next 90BP from a support factory only costs 1440 mass), your first 180 BP and next 180 BP for producing SCUs costs exactly the same amount. IMO this is a nerf for bigger teamgames, where you would have the ability to scale SCU production more, rather than 1v1 like you suggested. This means that in 1v1 situations, where the transition is likely to be tight and you won't have too much resources to spare, you can build the Quantum Gateway and have your first few crucial SCUs arrive on the battlefield in reasonable time. This also means that in larger teamgames where there are more resources, you cannot transition into super heavy SCU production as easily.
Unassistable Quantum Gateways are already relatively better for 1v1, but if you want to further make it more useful you can balance other areas, e.g., reduce the time it takes to build SCUs (from like ~60 seconds to ~40).