Space Engineers has a bit of a learning curve, but it’s definitely a nice sandbox-style game, just unfortunately a bit lacking in PvE content without mods.
Avorion on the other hand is much easier to get started with, but it’s also quite shallow in regards to story and such. The galaxy it generates can definitely be interesting though, it has a remarkably robust system for reputation, organizations, etc - and it applies between NPC factions as well.

I can personally also recommend Stationeers if you’re a fan of physics (in this case temperature/pressure/gas instead of movement) sandboxes, it has an even steeper learning curve than Space Engineers though, but it can also be loads of fun.