The problem with using slower than light systems is that you get absolutely ridiculous travel times. I mean, at 0.1 , which is fast, the closest star systems would be decades to hundreds of years away.
At .1c the trip to alpha centauri would take ~four decades.
But ships don't travel at a constant speed, .1c means months of acceleration and deceleration. The "accelerate until the midpoint and turn halfway" thing is actually wasteful and costs more fuel for a marginal increase in speed, but accelerating a fourth of the way and costing for a bit should work. Depending on your civilization's power output accelerating a small probe using a laser should be doable.
Also, remember the adage that a drive's effectiveness as a means of propulsion is directly proportional to its effectiveness as a weapon.
That's a really long time, looking at civilizations progress in the last 100 years.
That's the point. By the time the first humans arrive in another star system, who knows what Earth will be like if they even get back? That's part of the mystery!
The point is to make a game that spans many generations like EU4 or CKII. You have in-system problems to solve, but each starship is a huge investment. If you send a fleet to attack a star system, you don't know if the enemy civilization will still be there when you arrive. So not only do you bring your drone armies specialized for combat, but you also bring along ships for trade, colonization, manufacturing, etc, to deal with all the contingencies that could happen.
Taking over an interstellar empire could involve invading it through immigration or colonization rather than all-out war. And any single attack on a system is going to consist of in-situ construction and planning. You might have soldiers that, on arrival, turn out to be completely outmatched and decide to live among their enemies instead. Lots of things could happen.
You could also just shine your launching laser on an enemy star for a few years, then turn it off. They'll start to prepare for war by building combat ships to meet the ships they think are coming, only to realize they've wasted a lot of effort on just .
Perhaps warships won't even be able to decelerate by themselves. They'd force their targets to build braking lasers and shine them on the incoming warships to slow them down, or else the warships themselves slam into the system's planets and infrastructure at relativistic velocities. Warships would probably have to be automated, then, or at least be self-contained and manned by crew that don't realize the plan until they are told in the middle of the journey. The systems that emerge could be interesting.
It's something vastly different from what we normally get in space games, so I'm willing to try it out. If it doesn't work out I can always just put FTL earlier in the tech tree.