Just for fun, I made a rocket to a one way trip to Mercury, here is the plan:
It's less powerful, but more efficient, you'll just have to be patient, as the small engines have really low thrust. If you try it, the most difficult part is while getting into Earth orbit, you must aim for an orbit as low as possible, ideally below 50km altitude. Because of the 4 boosters, the rocket is very nervous at start, so it's easy to go too high. If that happens, you'll spend too much energy to circularize your orbit, so you would better start again. If you manage to do this, the rest is easy (provided you optimize at least a minimum your trajectory of course).
On my last try, I managed to get in close orbit around Mercury, and still had some fuel left (on the picture, the fuel gauge is the one of the last stage, the lander is still attached and have all its fuel):
I used the remaining fuel to brake, then undocked the lander, and then landing was easy. Actually I spent much more fuel than necessary because I wanted to land next to another lander from a precedent try
I don't know if that rocket will fit into your grid though, but I'm sure you will find a solution to this