The big question is, have we led campaigns without assistance from the other members of the Trust before? Are we likely to do so in the future?
If yes, we're gonna want to work on navy and air.
Rough outline of all the wars we've had...
The First Avernite War: Invasion of the Sons of War, Khornite Chaos Marines - Their invasion was relatively small. If such a force invaded now, we'd laugh at it. We won this on our own, and our naval support was small.
The Second Avernite War: Invasion by unknown Necrons - A sizable Necron fleet (13 cruisers and 29 escorts, which is a relatively large fleet for them, at least pre-Imperium). Using clever tactics with our own fleet we managed to wipe out half the escorts and half the cruisers, and the remaining ships were so damaged they took four days to hang out and repair their damage before landing troops. We then held out for another three days until the Vanaheim fleet came to finish off the rest.
Tyranid Splinter-Fleet Attack on Alfheim - Forces were gathered from all around the sub-sector and from elsewhere. This was one by a natural 100 on a high risk, high reward attack by the navy to destroy the enemy hive ships, but would have still come to a ground battle otherwise most likely.
Chaos Undivided Attack on Midgard - We arrived very shortly after the Chaos fleet did. Their main fleet was engaged with the Imperial Navy, and our small fleet managed to get most of our troops landed. However, the Imperial Navy had to withdraw and cede orbital control to the enemy. While they were able to do some bombardment, they couldn't do too much because all but one hive was under friendly control. We won the day by killing the Chaos Lord, disrupting all four sets of daemon summoning rituals they were doing. Without their unifying leader, the Chaos fleet split into separate groups and retreated.
Third Avernite War: Waaagh Garkill - The Orks initially sent a smaller attack against Alfheim, and were detected sending a larger fleet to Avernus while we were in transit. We sent Frederick, a small portion of the fleet, and some of the troops back to Avernus. We did not engage the Ork attackers in a naval battle outside of them fighting our static orbital defenses. They held orbital superiority for the whole war, but eventually left, likely when their forces in Alfheim were defeated and Garkill knew that the navy would be coming.
Dark Eldar Colony on Fjol IV - The naval forces had to get through the Dark Eldar defenses, and then we landed troops. We had orbital control for the most part, though the navy couldn't do too much to help. Our own segment at least won entirely through ground skill.
Basically, we can defend Avernus until the Imperial Navy shows up without much issue. Once we build up our planetary assault fleet we should have no problem landing forces, and Frederick himself can command the fleet while it's in the process of heading towards a planet, and then we can just use a Teleportarium to teleport him down to the surface when he's ready to take command on the ground. (again, Syr should be at home in that situation to take command of the forces on the ground if someone attacks) Our fleet can then either sit tight and control the orbital area around where we've landed or they can retreat if needed.