Get the 2600, the 1300X isn't good enough for all games to be played at ultra. It'll play most games, but it's too low-end for a gaming rig, especially when you can afford a 2600, there's no point in going with the 1300X then.
As fro the GPU, since you also have Freesync, I would say go with an RX 590 or RX 580 8 GB, you might not be able to play all games at ultra but all of them will run at high easily. At ultra, 75 FPS will not be stable, especially in very demanding games like the newer Assassin's Creed titles, Ghost Recon: Wildlands, etc.
If you absolutely want 75 FPS stable, then I would say get the RTX 2060, and you can use Freesync with an Nvidia GPU anyway, so you'll be fine. RTX 2060 will give you what you want in almost every game. Again, there will be a few exceptions, because ultra settings in some games require ridiculous GPU horsepower, while they don't increase visual quality that much, so compromises have to be made sometimes.