I recently received a 4k monitor. My NVIDIA 1660 Super is not powerful enough to handle the simulator in 4K. Therefore I set render scaling to 65. Performance is fine and the look of the scenery is also fine for me.
The only thing i noticed as a downside is that the glass cockpit instruments look blurry. They look more blurry than analog instruments in the cockpit or the outside scenery.
Is there a general problem with the render scaling on glas cockpit displays?
I just think it’s more noticeable when looking at the glass cockpit displays. But in fact I think everything would be a bit blurred when it’s being rendered at a resolution that is less than the native resolution of the monitor.
If you have got “lens correction” on - switch it off. That solved my blurry cockpit instruments. Good luck.
Unfortunately it has already been off in my case…so no improvement there. 
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Try render scaling at 50%. It may be that the 65% setting is forcing the graphics to try to render pixels where there aren’t any and it’s interpolating. I’ve seen this when users try to run a non-native resolution instead of using the display scaling function. It can make things so blurry that you’ll get headaches.
When I had my underpowered GPU (when I used to have RTX2060) and a 4K monitor, I ran 70% render scaling at 4K resolution. Anything under 70% gave me unacceptable bluriness of the fonts in the glass.
My monitor has a “Super Resolution” Mode. It more like super sharpening. 
When I have the renderscale aroung 65 to 70 the scenery looks almost a bit oversharpened but the glass cockpit displays remain kind of blurry. What is strange in my opinion is, that the non digital cockpit gauges look fine…only the glass displays are blurry. Well…seems I have to live with it until prices for graphic cards come back to normal.