A number of great systems have already been mentioned. Here are a few that I didn’t see mentioned.
Frontier Space - Retroclone of Star Frontiers
These bolt together nicely or are standalone
White Star - White Box SciFi
Warriors of the Red Planet - 0e SciFi
Colonial Troopers - 0e SciFi
Dare The...
I have never viewed the 5e boxed sets as being analogous to Classic/Basic D&D. The Basic D&D runs from 1977’s Holmes Edition to 1994’s The Classic Dungeons & Dragons Game. Basic D&D was more of a continuation of 0e than being a simplified version of AD&D.
A better comparison for the 5e boxed...