a note: for these we have different topics, as example:
https://forums.flightsimulator.com/t/after-playing-a-few-hours-fps-drops-from-40-to-5fps/
Important is to mention your installed mods, the region which you fly over, etc. See mentioned topic.
The OP in contrast reported a direct / overall decrease in fps. No time dependency etc…
EDIT : @Tweak112 ah, seen now you summary… yes… I refer to your mentioned point 2)