Reach on a limited budget is hard to achieve, glass prices grows exponentially, cameras with good bursts, buffer and AF are also more and more expensive. And all that hi pixels densities are ruined by physics and diffraction.
Basically, for less than $5000 the affordable long prime lenses over 500mm will open at 5.6, and the cheaper Tamron or Sigma zoom lenses will open at 6.3.
The problem is: on a 24MP sensor, diffraction kicks in in the red spectrum at F5, in the green at F6.3 and in the blue at F8. You also have to consider that any zoom lens like the tamron loses some sharpness and resolving power at the long end @600mm. That means the Tamron 150-600 at 600mm is allready not really providing 24Mp of details, and on top of that, diffraction creates a blurry halo about 25% (I won't go into technical details of the reduction of contrast in the modulation transfer function. Basically it's quite the same as applying a blur filter, I verified that in tests) in the red and 7% in the green, resulting in a photos with no more details than a 12MP full frame photo taken in the exact same circumstances with the same lense, and enlarged to match.
The supposed advantage of APS-C reach is totally lost because the sensor can't capture any more details than a 12MP sensor. Because physics. In comparison, on a 12MP APS-C sensor, the phenomenon that shows up at F6.3 on a 24MP appears at F8. Stopping down the lens to reach its best sharpness doesn't help: you loose more with diffraction anyway. Another factor is noise. All things being equal, cameras with small pixels make a bit more noise than cameras with bigger pixels. That just adds to the confusion and lack of details.
In conclusion, based on (a lot of) experiments and some (well, a lot too) maths:
- If you own a long lens opening at F6.3, you should use a camera with pixels bigger than 5 microns.
- If you own a long lens opening at F5.6, you should use a camera with pixels bigger than 4.5 microns.
To stay on the safe side of things, basically that means a 16MP APS-C or a 36MP full frame (that does 16MP in crop mode).
You just have to find the camera body that fits your AF, burst , buffer and ergonomy needs.