I’m back with good news!
As @ResetXPDR mentioned, the dev fps counter does not show FG generated frame, the Xbox toolbar counter shows double what the dev counter displays.
I’ve downgraded Terrain and Objects LOD from 200 to 150 but kept the rest in Ultra. I’m now getting 120 “real” fps (60 according to the dev overlay) in the same scenario as before. I still have to test with traffic and multiplayer enabled.
I will maybe double my RAM later but it’s not the priority right now as I’m happy for the moment.
Thanks again everyone for your help!