I've delayed getting cracking for a month already
Indeed as I said, don't delay, just think about the near future, and R5 3600 is a mighty fine CPU and, but it is almost written in stone that the 4xxx series of desktop CPU's will be a significant step up, and also the last CPU's on the AM4 socket which is what you are buying into. Grab a 3600, get your system up and running, enjoy it and in 12 weeks (guesstimate) drop in a superb upgrade that will last the life of the system, be that a 4900X, or a 4950X or whatever they are going to call it.

I'm still doing plenty of reading. It looks like it may be worth seeing what AMD has up their sleeve in terms of GPUs to rival the next-gen Nvidias. From what I can gather, it looks like AMD will have a 3080 equivalent, but not a 3090 equivalent. So, if the price jump between the 3080 and 3090 is a bridge too far I'll wait and see what the comparative results are like for AMD before deciding on a GPU.