This is what I finally decided to do. I went with a midrange Gigabyte X570 motherboard, 32gb RAM and a 3700x. I really wanted the 3900x but I think half the cores, and therefore the money, would have been wasted right now. But I can replace the 3700x in a couple of years with the latest gen and then hand the 3700x down to my son on his Tomahak Max. I did think about a B450 Tomahawk Max myself but thought I would get on the X570 platform for longevity and future new gen CPU's. Saving on the CPU meant I could spend a bit more on the motherboard and cooling.
I also have a new monitor arriving tomorrow so this is the first substantial upgrade I have made in a long time. I've had the monitor for 12 years and my old x79 for god knows how long. I couldn't wait for AM5 as the x79 died a few months ago.
I also have Thursday off work so will have a great day putting it all together