PossessedSonyDiscman

I'd go with the 3080, probably from MSI for all the reasons you mentioned. To be honest, with ray tracing being included in more and more games, and it becoming a thing, it is better to have it on board. If you don't care about ray tracing, then it's another story. DLSS on the other hand is a fantastic technology, that allows for respectable frame rates. Thing is, with games like Cyberpunk and other heavy hitting games coming out, DLSS is needed as rasterized performance of those cards isn't great enough to handle such games at higher resolutions, and upscaling is kinda needed, more so if you turn on rtx.
 
I don't know at which resolutions you game, but according to your needs, you could scale down to a 3070 if you wanna save a few bucks, esp at this time. That also is a fantastic card, and yes MSI is a good brand, but always see reviews of the specific card you're after if you haven't already.

Oh, okay then, 3080 it is...

Yeah, I forgot to add my target, it's 1440p and 144hz.
 
So I'm trying to find a good jump in performance, and since my card was a 5700XT, I wouldn't have that jump with a 3070. So my aim is, at least, starting with the RX 6800 models.

that would depend on how high the FCLK could go

generally 
3600 would perform better but the higher frequency will require higher FCLK ( if the interconnect can achieve it ) 

I have another kit with 3600mhz cl17, successfully overclock fckl and ram to 1867 and 3733 CL16 respectively. Now latency is 65.2+-1.

Correct me if I'm wrong, but isn't common knowledge that 1.45v is safe for b-die? Even 1.5v?
 
Thanks for the heads up, I had changed the primary timings and the motherboard did the rest. I just put tRC at 40 and tRFC at 288 and managed to get 53655MB/s. 

seems low, my 3600mhz cl18 has similar latency
 
also pls close all running task when benchmarking

it would be at it's limit  in terms of voltage 
if you can't do these timings at 1.4 keep it at 1.45 

Theoretical max bandwidth for 3600MHz memory is 57600MB/s so your results seem a bit low, maybe the secondary timings are really loose? 
 
Latency looks much more normal tho

Yeah, ryzen 3800x should suit you nice, it is a really nice chip.
 
With the 3700x i was able to easily use 3600mhz on my ram, while on the r7 1700 i was stuck on 2933, so i hope the same gainz for you too!

your motherboard is fine, just make sure to update the bios. ram frequency is mostly dictated by the cpu, not the motherboard, so you might have a nice surprise and the ram to work properly at its rated speed.

third gen Ryzen is better with ram speed

I would just keep the 370x. The ram speed can be couple of issues, maybe you dont give enough voltage, maybe too tight of timings?  But ryzen 3000 should be a bit better with ram speeds too. Also 2933 is pretty good already, what speed are you trying to achieve?
 
We are pretty much at the end of the Ryzen AM4 upgrade cycle, i wouldnt buy a new motherboard, especially for these prices. Keep the money for the AM5 socket next year.