I'm pretty sure every planet has been found at this point. This galaxy is close to 6 irl years old. No ones bothered to vote for a galaxy restart in a long time. So exploration isn't going to do anything at this point. Even then it is a pretty decent money sink that can potentially payout in monopolies and being able to secret away things like a large number of armies, but only in much younger galaxies.
Right now there is still room for people to make good headway in most spheres. If you want less competition you can look into psionics and geology. Geology can pay really well in the end and psionics is nice and stable.
Long game you can look into investing into politics and mysticism. These both require pretty large amounts of money to get started but can be pretty rewarding once you get established.
With politics you take the senator seat on a world and can then decide how the taxes on that world get spent. You can adjust it and even change how rich a planet is from here and even possibly, and illegally, pocket some. If we can get maybe 2 more players into politics we can even see about getting a new emperor to actually galactacily enforce taxes.
Mysticism also requires a large investment to get started. Though mysticism is strongest in that once a faith is entrenched on a world it not only pays well it is nearly impossible to peacefully cut away at that entrenchment to remove another's faith. Right now there are no controlled faiths and only the tiny remnants of old ones. I'll be changing that soon.