Do you really need more than 16gb now?
If you primarily do one thing at a time, like gaming, 16gb should be ok.
Memory usage via task manager will be misleading.
Windows keeps unused code in ram in anticipation of quick reuse.
To see if lack of ram is now impacting you, look at task manager/resource monitor/memory tab.
Check the hard fault page rate which is an indicator of ram shortage.
With sufficient ram, the rate should be zero.
A hard fault happens when windows needs to steal an active ram page.
That is not good.
If you will be streaming or other multitasking while gaming on the 13600K, then possibly more ram will be useful.
I think If in doubt, wait for the new processor and see how you do.
It is easy to upgrade ram later.
As to 3600/18 or 3200/16, I think I would pick the 3600/18 option.
Of course, if budget is not important, you can pay double and buy 3600/16.