Did you shoot them in jpeg? If you're going to crop a lot or do much post processing, 12-bit or 14-bit lossless raw is the way to go. You have much more flexibility in post. You also do not have to fight the jpeg artifacts that can cause issues when cropping. Steve Perry has a good video about raw vs jpeg in post production, though he's mostly concerned about color, exposure, and loss of details.
I agree with the previous too that 300 PF + 2x TC is pretty extreme. It will be hard to get super sharp results with that and the autofocus will only work well in the center AF point (f/8 equivalent lens).
The 500 f/5.6 PF would be the way to go, though it's a fair chunk of $.
Of you could consider the Tamron 150-600 g2 (or sigma 150-600C). It's a lot heavier than your 300 PF, but you'll get very good results at 550 - 600mm, especially stopped down to 7.1 or 8.