I think that depends on what you love about NMS.

If you’re a fan of the procgen exploration, Avorion, Starbound, or Elite:Dangerous

If you’re a fan of the multiplayer interaction, Eve Online or Star Citizen.

If you’re a fan of the base-building, Space Engineers or X4.

If you’re a fan of the Alien interactions, that’s very tough, but probably X4 or Star Control 2/ The Ur Quan Masters. xD

There aren’t a lot of other single games that have as many systems as NMS does.

I think that I would probably say start with X4 if you want 3D, and Starbound if you don’t mind 2D.