If you're looking for something that's less of a sandbox and more of a sim, Orbiter is splendidly enjoyable, and as realistic or as fanciful as you care to have it be.
And if you're looking for something slightly less a sim and more of a game, but still spiritually in the genre of "serious play" (especially with enough realism mods), I raise you... Kerbal Space Program.
I don't know. I had a 2+ year break from it until couple months ago; when first launching after the break, it showed me a consent popup... which I happily declined. Thank you, GDPR.
When KSP got bought by Take2 they put their privacy policy they use for all the games they publish on it. As far as I know KSP never did the tracking/analytics and considering how bad the reaction to the new privacy policy was I doubt that they'll start now.
http://orbit.medphys.ucl.ac.uk/