Well most of the DDR4 at 3600 i find, are with CAS 16 like the Crucial Ballistix 16GB DDR4-3600MHz , which are also a bit more expensive. The rest of 3600 modules i find are with CAS 19 , compared to the 4000 modules that i find with CAS 18. I assume i wont be seeing any difference with either of those, so my guess was to go with higher frequency.
As for the RX 5700, anything other than the Asus i mentioned, starts with 20-30 euro extra, so it increases also the difference compared to the 1660 and i start to wonder slowly again if it is worth investing 60-70 Euro more on the VGA. What i see though, is that the RX 5700 can draw almost double wattage compared to the Nvidia. That is an extreme difference.