Star Frontiers was pretty easy to understand. Pretty simple overall.
ANd it's available legally on Drive Thru in PDF and POD.
It does, however, put ship-operations into the realm of "high level play only" - the prerequisites for ship operation skills are beyond starting characters.
Star Wars d6 was more complicated, not sure if you would consider it more or less complicated than 5e.
It's less complicated, even in it's most complicated version (2.0), than D&D (any but OE without supplements). But, it's alos out of print and no legal PDFs exist.
D6 Space is the current version of those rules, but lacks the specific Star Wars content. It's available legally in PDF. It lacks a decent trade system
D6 Star Wars content can be played with D6 space with no alterations at all.
The original Warhammer 40K Rogue Trader (this is NOT the FFG game, this is from a MUCH earlier period, it is the first WArhammer 40K book that came out, had army rules and such, but also a RPG option in it) had an RPG option that was actually pretty simple overall.
Not really true. It was a wargame ruleset with continuing leader characters, but no mechanics for any kind of non-combat actions.
It's very much shy of the cross over to actual RPG. Using it as an RPG is possible, but involves either writing rules for all the non-combat actions one might want to mechanicalize, and/or going "only combat is mechanicalized," and/or being one of those "rules are only guidelines, so just declare a roll vs a stat and move on" types.
That latter crowd can do just as much RP using just about any minis game, as the rules underlying don't matter at all to that playstyle.
