Is the build mainly for MSFS?
If buying now - you would have to go via the second hand market to keep within/near your budget to better the 1660 Super. And, even then, you would have to look out for the odd local anomaly/bargain via social media for a 5700XT or 2070S.
Your other option is dropping to a 1080p panel (if wanting significant performance increase) - but this would be unfortunate if you think you would have the funds to upgrade your GPU in 12 to 18 months time. Plus, 1440p at 27" really is the optimal res/panel size and for future GPUs upgrades/gaming. And the 1660 Super would carry you through most other 1440p games in relative comfort until your were ready to upgrade. A 27" 1440p really is the sweetspot for res/size and if considering the above spec is a great foundation for a bigger GPU (First) and then a CPU upgrade when you feel it's needed.
*perhaps search MSFS gameplay at 1440p using 1660 Super or similar - setting compromises may not be that drastic...