Here's a tip:

Start with, at least, over a hundred warships, and more than that in cargo and passenger ships for economics and such.

I started with, oh, two dozen warships? They're each Dreadnoughts, more or less, but pretty much any of the other major nations could tear their apart without severe casualties. But my planets and my system would be next to impenetrable, so that's cool

And don't put a limit on speed. Space doesn't have speed, it doesn't care about speed. Acceleration- the heavier the ship, the slower it accelerates, the smaller the ship, the faster is accelerates. There is no 'speed limit' in space. On Earth, there is so much speed one can gain before it disintegrates or dies. In space, there is no air friction, there are no wheels. Sublight drives would make 300mph seem like you're standing still. FTL drives take you across vast distances nearly instantly- sublight drives speed you up and allow you to still see what's going on. 20% sublight is a much better speed limit than 300mph. Play Kerbal Space Program- in orbit, you go over 1000 mph at the slowest. 300mph is insanely slow. If that's the max you can reach in space, then your people shouldn't be in space.