If you mostly shoot with long telephoto lenses, IMO you'd be losing capability by going to a full frame body. All of the longer teles (beyond the Canon 55-250) are full frame. I understand there is a technical reason for that, but nevertheless, the same focal length tele (300mm or whatever) will produce the same size image on the sensor. If you're using a long tele to shoot something small and distant that you can't move closer to, the 1.6x crop factor is a real benefit. If you assume two sensors with the same number of MPs but one if FF and one is crop, the crop sensor will put more pixels right under the image of what you're shooting.
That said, I couldn't tell from your original images what focal length they were shot with. The 70-200/2.8s are popular lenses but maybe you want to consider a 100-400 (either a used Canon Mk I or the new Sigma Contemporary series). Both of those are around $800.
Maybe a higher pixel count crop body instead of an FF body. The 50D is 15.1 MP. I've seen used 60D's (18.1 MP) going for $400, and used 70D's (20.2 MP) in the $700-800 range. Personally, though, I think you'll get more bang for your buck with a long tele.