Steve, just for comparisons sake, how much warship tonnage did that NPR have to go along with the ground defenses? I am just trying to estimate the required build capacities.
It has about a million tons of warships. It is quite a large NPR with 1.5 billion population, 1100 construction factories, 56 research facilities, 245,000 tons of naval shipyard capacity and 1.1m tons of commercial shipyards.
It recently upgraded all its warship designs and is now building its first run of second generation ships. It can also upgrade its ground force designs, although it hasn't done that yet.
If you want to drop roughly equal forces, 500kt of troops, and assume your drop ships survive 3 runs, and assume your drop ships carry 50% of their tonnage, you need 300kt of dropships.
That would be currently 30% more military tonnage, but then again an equal sized empire should likely not be able to easily make that effort to conquer a homeworld. Every other colony will be much more lightly defended.
That is a tall order, but by no means infeasible. Putting on shields or using military engines will increase survival rate a lot, as double the speed means half the time under fire even without any lower to-hit chances from going above tracking speed, but maintenance and ease of building would likely still outclass any higher performance, which seems sad.
I think a buff to naval shipyards would make invasions possible, and simultaneously help larger ships, which seems to be a separate goal of C#.
Alternatively, increasing the cost of ground units some would decrease total formation sizes, and thus reduce transport requirements for invasions.