You're spot-on here. I switched to FF for a variety of reasons (low light, wide angle of view-14mm lens gets me 114° AoV) but it's very much the wrong gear for birds in flight, or other wildlife where what matters is the number of pixels on your subject.

They're tools. Good on you for picking the right one from your toolbox. Just because you have a fancy sledgehammer doesn't mean you should use it on nails.

Also: have you looked at extension tubes? that's a way to get more tele out of your lens - at the expense of effective aperture - and they're relatively inexpensive (nothing in this hobby is both usable and 'cheap').