12 GB 2400mhz to 16 GB 3600 mhz ram, what performance increase can I expect?

My system specs are:

I7 8700K @ 4.7 ghz

RX 6700 XT

12 Gb 2400 Mhz ram ( oc to 2600 mhz )

A month ago I bought the 6700 XT and while it performs great in some games ( RDR 2 for example ) other games it doesnt run as good as I would expect, so obviously I"m not running Dual channel on both sticks, it detects that only the 8Gb stick is running on dual channel so what can I expect upgrading to 2x8Gb so both sticks on Dual channel + 3600mhz Ram fron 2600mhz, could there be some ram bottleneck in my setup? I'm planning on buying the new ram in a couple of day and a new SSD.

Another thing is that I'm not sure what to upgrade next, I play on 1080 144hz right now, I'm thinking on buying a 1440p 144hz monitor so I could keep using the 8700k, or is it better to upgrade the CPU first?