.... You appear to be contradicting yourself < As highlit in bold. So why do you use FF cameras? Or are you meaning something else?
All photography has trade-offs regardless of camera-to-subject distance.
I use FF for the times where distance/FL
are a choice.
I also use crop mode/sensors/cropping and TC's when distance/FL isn't a choice... but I do not expect that they will make my images "better;" I expect them to make my images "worse."
FF is not better when distance/FL is not a choice because:
Larger sensors receive/record more light
when the recorded composition is the same. If the composition is different with the subject the same size on the sensor, then the larger sensor still records more light. But that "more light" comes from areas not included on the smaller sensor... and when you crop in post you throw that away. Using a crop sensor is effectively the same thing.
In order to get the same composition you could use a longer lens... but longer lenses are exponentially more expensive, and they have slower max apertures. Which basically means you have to use a higher ISO due to recording less light, or you have to make some other sacrifices.
You can also use TC's to extend the FL. That also reduces the max aperture (size). Plus they introduce their own optical errors (softness, CA, etc). Which again means using a higher ISO or making sacrifices.
And longer distances (increased magnification) also introduce environmental factors and an exponential increase in demand of technique, both of which can be extremely difficult to overcome.
The light per pixel consideration/debate is mostly irrelevant... it only relates to how an image appears to you on your display. It has little relevance as to how an image will appear to someone else on a different display or when printed. What actually matters is light per area recorded... ISO is not light/exposure.
*Edited to remove DOF considerations/error...