Your not need ddr5 for m.2 support plenty of mobos do nvme drives in x370 x470 x570 i'm on an x370 have 2 nvmes drives planing on getting a card to add a 4tb into a 1x slot as i'm out of other options to add more storage myself.
at that kind of budget look into the 2nd hand market on a last gen card depending on what you wish to play and what settings, but something like a 6700XT 3060ti I'd expect you could find one under that or even take the plump and go Intel A750 as they sell new below that price might leave you with some left to go towards the other things in future.
depends on how much you want the loading speed but i'd say hold off and do it the next time with cpu as prices will have fallen more on everything in that zone where as GPU prices are not going to fall much at all.
3800x isn't a bad cpu and as much as a mobo might cost £100 that cuts you short on options for the GPU to have something that will last a time, that'd be my thoughts at least.
GPU is where your gain most for games, it'd be night and day difference for that.
but i'd say a new mobo might cost you £100 maybe less and then storage for that, you don't need to get the latest and greatest or new just something that works and just be sure to get from someone with 100% feedback, with them cards being not so old your run into less issues just don't buy something that looks ex mining or is being sold for parts mistakes can happen.
imho sometimes nvme doesn't seem that much faster than a 500mb ssd tbh that runs off sata, I only use mine with games i play often or that requier it, nvme in terms of direct storage still isnt much of a thing yet, but for the OS you'd see a jump over a normal ssd.
