There will never be a significant difference between the 12700K and 12900KS for games, a fancy motherboard is great if you plan on filling up all of the slots with drives etc. but otherwise will net you no extra performance, and the Alderlake chips can't run 4x DDR5 DIMM's at decent speed at all, so you'll end up stuck with something slower than DDR4 if you want to add more RAM.
Just over £600 will get you the 12700KF/Z690/32GB DDR4 3600, I mentioned before, that is less than the cost of the 12900KS on its own, you could just stick the savings made to one side and get a proper long term platform in a couple of years with the money left over. We aren't in the 2010-2017 period with total stagnation in the CPU market, so changes are happening much faster, and for now buying cheaper and more often is actually the mores sensible option presently, even more so with brand new RAM and PCI-E standards.
Obviously you can do what you like, just advice from someone who works in the industry, and builds more systems than I have hot dinners.
