I found that 2000mhz FCLK with good timings and the 2200mhz GPU @1.15v Soc was a good compromise. The Corsair bdie kit here could probably keep going, but vDimm is limited at 1.35v.
2000mhz CL17 is about as fast as the 2100mhz CL18 level, but more stable for the FCLK & GPU.
Nice tweaking of your Corsair Vengeance 3800MHz CL18 kit, I myself have essentially the same type of kit as you: the model above, that is to say the Corsair Vengeance 32GB(4x8GB) 4000MHz CL19 kit which I uses only 2 memory modules like you (it's a shame that we don't have 4 DDR4 SODIMM slots on our ASRock X300M-STX motherboards, we would have 32GB of Samsung B-Die High binned). I wanted to know: Did you try to push higher in frequency than 4000MHz CL17 and 4200MHz CL18? For example 4400MHz or even more? And if so, do you have any screenshots of your timing settings under "ZenTimings"? Because I am in the process of completely reassembling my configuration based on DeskMini X300 motherboard in another box (Akasa Cypher ST to which I have made some modifications) and I have gone from a Ryzen 7 PRO 4750G to Ryzen 7 5700G because past 4000MHz on DDR4 on the 4750G, its Vega 8 iGPU sees its VRAM speed stuck at 2000MHz, which is not the case on the 5700G.
Another thing, I took a Crucial Ballistix 32GB (2x16GB) 3200MHz CL16-18-18-36-1T kit in addition to do overclocking and latency tweaking tests because apparently these memory modules would be equipped with Micron E-Die chips which are reputed to go up fairly easily to at least 4000MHz in CL18-20-20-... after seeing if they can really get there, see pushing further? But if it was really the case, it would finally allow me to go from 16GB to 32GB of total RAM, anyway I would see... otherwise I saw other rumors if a Japanese forum mentioning 32GB kits (2x16GB) of OEM/generic SKHynix 2666MHz equipped with SKHynix CJR or DJR chips which would support operating at 4000MHz or more with latencies between CL18-21-21-... and 19-22-22-... under 1.35v of course.