12 threads on a cpu is going to last a good long time. Figure at least as long as a 4/8 quad has been viable. Game devs won't change requirements too fast or they cut out too much of a competitive market.
By the time the 10600k becomes thread obsolete we'll be on 13th/14th gen cpus and DDR 5, so Op would be looking at a platform upgrade anyways.
Mixing ram is never a good idea, and 32Gb is more than sufficient for anything except multiple virtual machines, semi-prossional rendering and content creation etc.
You really don't want to run the 10700k+ on a more budget board if there's plans to move up. Should bite the bullet and get a good board from the start.
The 10600k gets almost identical fps at boost speeds as the 10700k and 10900k at the same speeds because games aren't using more than 6-12 threads and the threads aren't saturating the bandwidth of a single core + HT. So the extra cores aren't much of a benefit over the HT.
It's a lot of money for very little (if any) gains as you are more likely to be gpu bound than cpu bound. Once you get beyond monitor refresh its all a moot point anyways.
If you are after benchmarks and just want the bragging rights of saying 'hey, I've got money to waste, so now I own the fastest cpu and the baddest gpu on the planet', then by all means go for it, but that last 10% almost doubles the price of the pc and at the power you are dealing with shows very little actual value, especially gaming on anything less than 4k/5k.
Id prefer to add a second monitor, or even a third, but 2 is generally optimal for most uses. Or as an expensive option, go for something like a Super ultra wide, it's 2x 27" monitors in one chassis that can be windowed as you please.