Something to keep in mind no one has mentioned, if you and this friend make a base together, but, say you keep building when they go and log out, more times than not, this messes up your base up badly if you used the terrain tool as their save will have the terrain untouched, while yours has it flattened etc. It glitches reallyyyy badly. Even if both of you upload base data.
The game tries to basically load both versions of the save, one with untouched terrain, and the modified terrain
Be sure both players are “at” the base and online before either person even touches the terrain tool.
Out in the world its fine and no big deal, but using it at base if only one of you is online messes things up super bad, it becomes extremely apparent the bigger you build your base. One of mine is completely fine until she logs in, once she does i have mountains in about 50% of my base, and re flattening them doesnt help, they pop back in 10 minutes later, sometimes less.
I would try to just stick to land already flat in all scenarios honestly. If you have to use the tool to flatten terrain its not a good base idea in NMS