And then, eventually, you'll have done everything in the game... And then you'll install RSS+RP-0 [0] and struggle to get back into orbit again. The mods add a ton of depth beyond what the base game can provide.
For those curious, RSS stands for Real Solar System - replacing the Kerbol system with ours. This is a large difficulty ramp, since things in our solar system are far larger than those in Kerbol, so even with the same surface gravities delta v requirements end up much higher. (For instance, ~2.7km/s is enough to hit orbit on Kerbin if flown properly, while 9.5km/s is more common on Earth) To partially compensate for this you have RP-0, a modpack that (among other things) replaces all the engines, fuels, tanks and other parts with their real life counterparts, making the higher delta v requirements actually achievable. (And also adding a ton of complications, such as life support, communications delay, cryogenic fuel boil off, limited restarts, fuel ullage, and more!)
For those curious, RSS stands for Real Solar System - replacing the Kerbol system with ours. This is a large difficulty ramp, since things in our solar system are far larger than those in Kerbol, so even with the same surface gravities delta v requirements end up much higher. (For instance, ~2.7km/s is enough to hit orbit on Kerbin if flown properly, while 9.5km/s is more common on Earth) To partially compensate for this you have RP-0, a modpack that (among other things) replaces all the engines, fuels, tanks and other parts with their real life counterparts, making the higher delta v requirements actually achievable. (And also adding a ton of complications, such as life support, communications delay, cryogenic fuel boil off, limited restarts, fuel ullage, and more!)
0: https://github.com/KSP-RO/RP-0