Honestly, I would save your cash for the moment. I don’t have an AMD based PC, but for MSFS 2020 in particular the X3D processors offer much superior performance than either your existing 3600 or the alternative proposed 5600.
If you’re going to go the route of selling the old PC & getting a new one, then I don’t think it’s a great idea to go for another AM4 / DDR4 build. You already have that mobo, SSD, RAM and PSU, so that’s spending quite a lot on a sideways move which is unlikely to be significantly better than what you have now, and is stuck in the previous generation.
Given what you’ve posted already and thinking mostly about use with MSFS 2020, I’d personally be tossing up the following choices:
1 - Either keep the PC you have, and use the budget on the bits that can be usefully and significantly upgraded: 5800X3D, 4070 ti/Super, 32 or 64 DDR4 RAM in two sticks, a new 2TB m2 SSD & a suitable PSU. You can keep using your existing mobo, case (& the GPU for a while if necessary), and also keep your existing 500GB SSD just for Windows & drivers etc. This builds in a bit of future-proofing, and will make any later changes easier. It also matches the spec of your mates stutter-free PC mentioned above.
Or alternatively:
2 - Hold on and save up a little more for a 7800X3D, AM5 mobo, 32GB or 64GB DDR5 RAM, 2 x m2 SSD’s (1 for Windows & 1 for games), 4XXX or 5XXX GPU (depending on required screens & resolution) and a suitable PSU. You don’t need to spend loads on a case, RGB strips & fans etc., and you’ll be fine with an air cooler rather than a liquid AIO on that X3D chip. You can also offset a bit of the cost by selling on the old PC.
This build will be way better than either your mates pc or the sideways-move 5600 build mentioned above. If you go for something in between then I don’t think you’d be getting good value, or a noticeable step up in performance.