I would get the Ryzen 5 5600X. If you can’t wait (I would wait a couple months if necessary), get the i5-10600K. Just don’t get any Ryzen 3000 series (R5 3600X / R7 3700X). The Ryzen 3000 series is clearly behind Intel in MSFS (and now Ryzen 5000 series) by a good margin, especially on the resolution you are playing.
Both the Ryzen 5 5600X and i5-10600K are within 30$ bucks difference. I’m not sure about motherboards, but for Intel you need the Z490 to overclock the i5 down the line (it is worthy). Ryzen 5 5600X gets the priority because it is way ahead Intel now and don’t require expensive cooling solutions, even when overclocked.
Regarding GPUs, prices are all over the place, so check around the RTX 2060 (Super) and the new 3060 Ti. I think the 3060 will come just in 2021, perhaps even the 3060 Ti.
Regarding AMD cards… I would not put money on it. Driver support does not come near Nvidia last time I checked. I did have a Radeon in the past. Would never buy it again unless we go through a few generations of very stable driver support.
The ideal is 32GB of RAM. In your case, 3200Mhz CL16 is a good cost benefit. But you could get 16GB now and 16 later. Just don’t skimp on clock and latency. Get good RAM, especially for Ryzen.
Hardware Unboxed was stating that 4 RAM sticks gives a bunch more frames in games over two sticks. It might compromise some of the overclocking on the Intel part, by I don’t think it would affect Ryzen, which does not overclock well anyways last time I checked.
And I would get a 1TB NVMe for Windows, the most important software you use and MSFS, and a 2TB HDD for your personal files and other stuff.
Don’t go ballistic on the PSU. Get a good Seasonic 550W / 600W and call it a day.
Since the R5 5600X is on the price bracket of the i5 and is the best gaming CPU at the moment, I would do whatever necessary to get one. As I said, to get a R5 3600X at this point would be suicide. The Ryzen 3000 series has latency problems and such. In 1080p and 1440p, it trails Intel and Ryzen 5000 series by a lot. If you can’t wait, get the i5-10600K.
A top notch CPU (albeit somewhat cheap), good RAM and a solid PSU is a good base to stick a GPU to game.