Re: My mostly hard sci-fi campaign
Rather than focussing on speed as controlling the transit time of interplanetary ships, you might look at thrust divided by mass, or acceleration. Divide the distances by two, assume accelerate full-time the first half, decelerate full-time the second. This will, I think, compress the travel times between distant points while retaining a pretty firm minimum transit time between near-Earth stations.
If you have the patience for it, you might get heliocentric maps of where the planets actually will be for the time interval of interest to your campaign. I don't have immediate suggestions for where to get those, but after class I can scare some up, I think, if Google doesn't find 'em for you quicker.
An added complication can be that the Sun is several degrees wide in the radio ... that is, radio communications between planets can be blocked by the Sun for a few weeks when they are on opposite sides of it. Now, the existence of multiple stations around the Solar System means you have alternate communications paths, albeit with longer light-travel times.
On the Moon and Mars, you could posit that deposits of ground ice are an important resource that would be contention points for the ground stations there. On the Moon, those might be expected to be in the polar craters. On Mars, it could be more local, dependent upon purely local geology with a secondary effect from latitude. Also, on Mars, there are reliably predictable seasonal global dust storms that happen and blanket more or less the whole planet for months at a time. Nothing like climate to complicate your politics, and give opportunities for military action.
Finally, forecast the 11-year solar activity cycle out into your campaign period. If you have solar max happen in a game relevant era, use it.