I am an avid player of 5e D&D, but I want to start going into other systems, I already own the Pathfinder core rulebook, and I have used the open reference, so Pathfinder is out of the picture. I was thinking about three games in general, if you have played them, tell me about them and what you think.
Games: Conan, an age undreamed of, Starfinder, Call Of Cthulhu
Conan's system, while not bad, has a potential snowball effect to be wary of; it's present in all 2d20 system games. For me, that is a fatal flaw; one that I noticed in the Star Trek Adventures playtest. If the players start rolling poorly, it's all too easy to wind up with penalties stacked so high that success becomes nigh-impossible. The "Increase Threat Range" Threat spend is very potent, and the primary cause for it. Otherwise, it's a straightforward Narrativist-Gamist hybrid approach.
Starfinder is pure gamist... Excessive complexity for what appears to be a kitchen-sink kind of setting. I have not played, and would not consider playing, Starfinder simply because of the excess complexity. I have read it. It really is Pathfinder 2E in space. If you dislike PF, SF is not going to be much better. (The hit point system is slightly different, and so may be sufficiently different, but I doubt it.)
Call of Cthulhu is a fairly simulationist game. I dislike the Cthulhu Mythos immensely, but the system is pretty good. If you're looking specifically for the Mythos elements, get CoC; otherwise, get BRP instead. Same rules, no mythos, and a wider range of starting gear.
Based upon those three - I'm going to suggest trying the preview module for Conan - it's free, save for printing it out (if you're one who needs paper), and it's got enough of the system to let you know if you like it. If so, then buy the full version. (if you buy it dead tree, Modiphius does participate in Bits n Mortar, so you can get the PDF for free)
You might consider also Mutant Chronicles - also by modiphius. Same issue as Conan - the potential Thread-spend snowball - and has a similar setting feel to both Starfinder and Warhammer 40k