You don’t appear to be listening to what people are telling you. You’re new 4090 will be next to irrelevant if you are CPU bound. As will a 5090 or a 6090.
You need to find what your max fps is with Frame Gen on. That means HAGS on. DX12 on. Riva Tuner installed. Once you know that you can then cut your cloth to suit.
Example. I can get between 80-120 fps with frame gen on. Do I need that? No, so I cap my frames to 60. This relieves pressure on my CPU and allows me wriggle room to increase other settings. A scattergun approach isn’t going to help much.
Here’s a thread were someone else had forked out on an expensive 40 series card and was a bit disappointed (or so he thought) with the results 
I gave him a few pointers and Bingo!
Rtx 4080 frames! - General Discussion & Community Support / Tech Talk - Microsoft Flight Simulator Forums