Any modern cpu will work with dx12, what's more important is that the os supports it (win10), the gpu supports it and the game is written to use dx12. Hardly any games around with dx12 support and while your gpu does support dx12 it's really underpowered for most gaming situations.
I realize you don't want to debate your gpu but it's a large part of the gaming experience. It won't matter much whether you go with an i5 4690, 4690k, 4590, 6600k, fx 8350 or fx 6300, that gpu is going to likely hold you back. For games like csgo, dota2, lol etc it will do ok, for most anything else like gtav or bf4 it's going to struggle with the lowest settings at 1080p.
Out of the cpu's, I'd go with an i5. If you're on a tight budget, nothing wrong with a 4th gen i5. It will depend on prices where you're at, my point of reference are usd pricing but cpu's could be priced differently in your currency and in your country. Intel cpu's are almost always better performers in gaming in a wide range of games due to higher ipc (per core performance). If there's a good deal on an fx cpu and you're on a tight budget, they'll work.