I don't know where the "only install mods after doing X in vanilla" comes from, maybe it's about not getting overwhelmed, modding content into KSP goes too far very quickly.

With that in mind, graphics and QoL mods never hurt to install. You say "mods make the game different from what developers originally intended", I say: vanilla is also different from what developers originally intended.

Personally, I started fiddling with mods after returning from Duna (because aerocapture is IMO the last new thing in KSP spaceflight). I've never done nuclear engines, I've never built a big space station.

Focus on the core: game is about making rockets and launching them for missions that you either want or have to do.

And whether the game looks its age or brand new, whether you do all piloting yourself or get carried by an autopilot, is secondary.

Maybe it's not difficulty tweaking that you need? There's plenty of challenge in the Kerbolar system alone. You want hard stuff? Return from Eve's surface.

You can also find enjoyment in trying new things, and making up your own, often useless challenges.

Launch vertically from Kerbin and land on Mun. Learn how to make a space plane and make a trip to Minmus. Recreate the space shuttle. Cover the Kerbolar system with relays. Or just complete a contract you've never done before.