PCIe Gen 3 or 4 doesn’t matter currently as they are not utilized enough to make a difference- in the future maybe.

3600 CL16 has been the sweetspot for AMD Zen 2 and tuning in either timings or MHz has have relatively small performance effects - they were there but only small percentage change which may only show in benchmarks and not games. Now the Zen 3 has had a marketing slide leaked which indicates to better FLCK which could change the sweetspot for RAM. My suggestion is to wait until you get some reviews before buying RAM if possible, you can go ahead with mobo, cooling solution, storage and so on. If not 3600 CL16 is probably a good bet, check if the kit you intent to buy is on QVL that should help you get XMP (DOCP) to work easily if you are not into fiddling with RAM settings.