I would definitely recommend to change the 5700x to a 5800x3D as MSFS works best with the additional vcache. This should not cost too much more so you could keep the 4070.
At 1080 you are more likely CPU limited than GPU limited (check for yourself with MSFS developer mode FPS view), so a CPU upgrade only at this time would make more sense. As @WingWarper1 says, if you can stretch to a 5800X3D that would give you the best experience at 1080.
Check if your motherboard supports 5800X3D, and if it does, replace your current CPU with it first.
If you are budget constrained, keep in mind buying a used CPU could be an option.
As people move to the latest gen AMD chips, I’ve seen pretty nice deals on perfectly working units, still under warranty, on the used market.
4070 is probably good enough for 1080p, and has Frame Generation.
Lately AMD has been selling 6950XT GPUs like hot cakes at the same price brand new, in the US (599$ + tax).
The latter draws twice the power, has slightly better raw performance, but more importantly has more VRAM, which comes in handy for 4K.
On the other hand it does not have the DLSS/FG/Ray Tracing features of Nvidia.
At an even lower price point, RX6800 is another budget option with 16gb of VRAM.
The additional VRAM is very important esp. for DX12 in MSFS if you want to try 4K or 2K.