I remember reading that the original plan for KSP2 was to introduce colony parts around tiers 4–5 of the tech tree. However, I think adding early (think Jamestown from For All Mankind s1) base-building parts around tiers 2–3 would be more realistic and better for progression.
Parts like small habitats, early science labs, and even the ability to convert large fuel tanks into wet workshops would allow players to build their first Mun bases while beginning interplanetary exploration. It would mirror real-world logic — agencies wouldn’t wait until we’ve explored the outer solar system before building a basic Moon base, especially in a "space-utopia" like KSP where there are no politics inhibiting space exploration.
Delaying all colony parts to tier 4 or later creates a gap where players can visit planets but can’t establish even rudimentary infrastructure. Early base parts wouldn’t have to be full colonies — just enough to feel like a stepping stone toward them.
NOTE: My original version of this text was not very concise, so I had ChatGPT polish it a bit, but it's all still my feelings and ideas.