So this following suggestion is mostly sarcastic:
Make T3 Support HQ Land Factories Cheap. Like Super Cheap and Fast to build. And better BP. So deploying and setting up a T3 Land Proxy can be done quickly, cheaply and efficient. Perhaps increase of T3 HQ's Land. Half the problem as folks said is travel time/deployment of T3 Units in mass. If you make it 'easier' to setup a late stage T3 Land Proxy Base, so you can bump out units and transition to units there faster it might be easier.
The topic of defenders advantage is a failed T3 Push, while bad onto itself, is only made worse with reclaim. So having reclaimed values as you tech up get scaled down/I dunno. But words let me rephrase:
One of problems of land is end game, both travel time to deploy land forces (can be scouted and responded too), and ever increasing mass, a failed push gives an opposing player. The deployment is due how for variety of reasons important buildings like T2+ factories tend to be in someone main base. And then despite engi mod, I noticed (once again shitty 1k global) still tend to hyper assist hyper tech factories with engis vs support factories. And to do that you need the BP, which tends to once again be in your main base.
In addition to this piece of trivia. A T3 Engi building a T3 Support Factory takes almost same time to build as T1 Engi building T1 Land factory which then upgrades itself to T2 Support Factory. (Also a T2 Engi Building T2 Support directly is slower than building T1 Factory then upgrading T2 Support. And T1 Engi building a T1 Land which upgrades to T2 Support takes same time as building T2 Support directly as T2 Engi).
Words this shitty 1k global thinks, maybe it worth considering looking at relationship to support factories and HQ's again. If part of the problem is deployment of T3 Units (if current status of T3 mostly as defensive tool is not desired) to battlefield. Is looking at removing the deployment aspect of T3 in some shape or form. Making T3 Support Factories cheaper to build, deploy and utilize in mass. Another aspect of this while raw cost is important too obivously the drain of unassisted support factories. Reducing there raw BP by half. As that make their eco per second comparable (when unassisted) comparable to T2 Land factories.
This would make it easier to deploy T3 Units using support factories and makes going from a T2 Factory to T3 Factory not result in a significant increase of economic drain. A similar type of relationship could be done for T2 Support factories and T2 HQ factories with build power, mass and energy cost reductions. Or atleast make T2 Support build straight from T2 Engies faster than T1 Factories built by T1 engi upgrading to T2 unsupported.
Also if curious its faster to build a T2 Factory with a T3 Engi then support it to a T3 support vs building a straight T3 Support when built by T3 Engi. (Upgrade Commanders building support factories are faster than lower tech + upgrading). Words.
Basically; maybe adjusting cost and build power of support factories?