AlexUP

That should be impossible.

you can't damage your pc with that... 
 
gtx 1660s can render much faster at very similar quality to cpu rendering but not all video editing software has support for nvenc

CPU rendering is slower but sometimes leads to best quality. GPU rendering is way faster but the end result may be subpar.
 
As said above, there's no such thing as damaging your PC just by  making full use of it.

If you don't apply XMP all the sticks should run at the base frequency, which is 2133. If all run at base specs different kits should work together.

Base line for ddr4 is either 2133 or 2400.

All of them will run at 2400, for the most part it should work
Tho mix and matching ram isn't always guaranteed to work 
But for the most part you'll be fine.

yes

it might be ram, but it surely is your cpu as well that can't handle multi-tasking.
as above, look in task manager. if your ram is not close to 90% utilisation, but the cpu is, a ram upgrade won't help.

You can tell by looking in Resource Manager - Memory then then the "Used Physical Memory" graph

16GB of RAM really is reccomended these days, since there are many games that'll max out your 8GB when windows and some background apps are running. The upgrade could be worth it.

Yeah, i get around 100-110 FPS on MW without ANY problems at all, and the game runs super smooth, no stutters/lag spikes etc.

I don't know if strictly the RAM upgrade will fix this issue, but it certainly won't make it worse. I was actually quite surprised that you mentioned that Modern Warfare was working fine, as I've heard several people saying that it runs very poorly with just 8GB of RAM. Either way, 16GB has kind of become the standard nowadays, and it will greatly improve multi-tasking and some games will definitely benefit from having the additional memory.

you can oc your cpu if your motherboard allows it.
8gb more ram helps a lot.

Right, well still, unless you're playing at 144Hz, I would try limiting the max FPS in-game to prevent the CPU from trying to output unnecessary frames. You can also try to increase graphics intensive settings in-game, which should also sort of balance out the CPU/GPU loads.
Either way, as @glenalz81 mentioned above, a CPU and RAM upgrade would still obviously be the best solution.

Yeah I was expecting that. Try limiting your framerate in-game and slightly overclocking the CPU, if your cooling allows it. Also close other CPU intensive apps in the background that you might not need.

What is your CPU usage while in-game and you notice your sound cutting out and/or lag spikes?

Alright, thanks! Found out that i can get up to 64GB's maximum.

if you were going to upgrade, First check your CPU Specifications for the Maximum amount of memory it can hold.
Other than that just get the RAM sticks after what I told you Above this text.

yes, but make sure they are the exact same kit

My i7-8750H is jealous...

those temps are fine, start getting worried around 80-90c

Perfectly fine. 85-95C is hot, above 100C is dangerous. You're well below that. 

Both have the same memory and clock speeds AFAICT so pick whichevers cheaper/has better design

palit

Two fans if you like silence.
I recommend MSI Gaming (silent and nice), but some may recommend other brands.
Personally I don't like Gigabyte.