If you're going to throw that out, let me know which dumpster. I'd love to drop-in replace my 4690K from one of my old machines and upgrade the RAM. 😉
4790K is still a decent CPU, even if it and the DDR3 memory are starting to show their age. Definitely still suitable for 1080p gaming in most cases. My recommendations are exactly what you've already suggested:
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Try Linux. Games objectively perform better on linux than windows now, even with the compatibility layers. Windows is now the biggest bottleneck in peoples' PC builds. Debian works. I recommend choosing the KDE "flavor" of Debian instead of GNOME, it will be a more familiar experience.
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Try the OC for the CPU and also for the RAM. My 4690K overclocked really well. Its been a long time since I've messed with that, so I'd say just find some guides online specific to Haswell.
The nice thing is that both of these recommendations are free to try. You can see if they get you back to the peformance you are happy with, and if not, then you'll know you're right to think about replacing the CPU.