> If this remains a long term design limitation
It is not a long term limitation. One might design any number of stages to fly in place of Starship on top of the Super Heavy booster.
Also, large science payloads can be launched by SLS. Back in the days of the Ares program, there were proposals to launch an 8 meter reflector (ATLAST-8m)[1] using Ares V. SLS anticipates payloads of such size as well.
So there are at least two viable platforms on which enormous space mirrors might launch, and one of them is likely to be highly cost effective. It's up to the science establishment to propose such missions and get them funded.