Taking Terra is not easy at this time. While there are less Com Guards to greet a landing there is still very much a SDS ready to remind you why the Star League Defense Force Navy was cut to pieces securing the initial landings and they had hundreds of Warships.
The ComGuard Navy is also smaller at this time but Luyten is either in the Terran System or a single jump away and Comstar does have active Warships at this time smaller at this time still beats non-existent like all of the Successor States.
And say you think of all of that and brought enough Nukes to deal with Comstar's naval supremacy, there is the fact you had to use nukes in space just to get the opportunity to land (and get lucky doing it). Comstar is not a third tier power technologically they will be able to respond with nukes of their own in short order either against your forces enroute to Terra or at their landing sites. And can even say "What we never signed the Ares Conventions of War and the Blessed Founder was quite clear on what an invader of Holy Terra could expect.".
This is all I might add with your invasion being a complete surprise because some how ROM was doing it's best SAFE impression.