It may be the "first of the next gen of "flight sims, but it certainly doesn't represent the whole pantheon of games or even game engines. MSFS, like most new AAA games, has performance issues. Those issues seem to be independent of the hardware available. When 3900X's with 2080s have massive frame rate dips, there's more going on than "the hardware isn't good enough". That's an issue with game engine performance scaling. That the difference in overall performance of the 4700 and a system with the "minimum" spec of an RX570 is so minimal, it tells you right there that something, somewhere isn't scaling properly.
I will conceed one very important thing with MSFS, it is VERY texture heavy. It absolutely churns through VRAM and moves a lot of data back and forth from system RAM to VRAM. Since APUs still don't use a full unified memory model, there is going to be a lot of duplication of data in RAM, and a lot of needless copying back and forth, further clogging up the memory bus. I can suggest that, if you are going to attempt to play a game like MSFS with an APU, memory tuning and size are going to be crucial in getting anywhere near playable performance.