The RTX 3000 series cards are like gold dust. What CPU have you gone with as well as that could also be slowing things up.
One day the PC market will be normal again.... Maybe
For £300 you can get the Samsung G5 which is your best bet. You'll be looking around the £450 mark for an IPS monitor at 1440p/144hz :)
https://www.samsung.com/uk/monitors/gaming/wqhd-gaming-monitor-with-1000r-curved-screen-27-inch-lc27g55tqwuxen/
https://www.pcspecialist.co.uk/forums/threads/amd-vs-intel.72675/
Some great reading there regarding AMD/Intel at present
You will get slightly better performance out of the 3090 than the 3080 but even in MSFS your paying £700-800 more for 10-15 extra FPS. Of course its your choice and money...
With that monitor Flight Sim will be a dream.
I would recommend, switching to an AMD build if possible, you can also save a lot of money and go with the 3080 rather than 3090 and get fantastic performance still. (Although you might have to wait longer)
You would need a better case for sure...