Try using this as your system to start off with in Career mode.
Every planet in a highly elliptical, inclined orbit
Have them all be on different sides of Kerbol
Change moons around and planetary settings for lower gravities
Make them far enough apart that you really need to be planning if you want to properly intercept a world
Only use the first few tech tree levels (5-ish tiers)
If it's too hard or easy let me know because I want a reasonable challenge.
Thanks!
-Noah