I have a very similar system 5900x/3080. You’re probably in the same situation as a good chunk of us simmers unfortunately, where we’re all mainthread limited by the CPU. Hence why when you mentioned changing your resolution you get none to very little FPS increases. That’s because your GPU can handle it but your CPU’s mainthread can’t. I’m same situation, My native is 1440p but upping to 4k I don’t really loose any performance in terms of FPS but get more stutters if anything.
But from the FPS you’ve mentioned depending on where you are and what airports you’re at and if other players are about and or if you run AI traffic/Vatsim. 20-35fps is about where I sit most of the time. Sometimes even dipping below 15fps when at Heathrow Inibuilds scenery with the Fenix A320.
I tend to cap my FPS via the Nvidia Control Panel to 35fps and that helps a little with the micro stutters, with the PMDG aircraft anyays, sadly the Fenix for what ever reason is a bit of a stutter fest at large aiports for me (Heathrow, LAX) To which I have to sadly avoid when flying the Fenix a320.
Other than potentially picking up a 5800x3D/13th Gen Intel or waiting for the 7000x3D’s, along with remortgaging your house and getting a 4090 to brute force your way through. Not much else you can really do in terms of raising your FPS.