A longer lens would help so less cropping, i hate to say it but a much later camera with more MP and better iso handling plus better DR.
If you have the money steady hands and want to do much BIF my basic Nikon recommendation would be D7200,300mm f4 and 1.4 tc.
Would like to see some of your static bird images that you are happy with as it would be easier to recommend kit to beat it.
I opted for the D500 and Sigma `100-400 as i like to do BIF but for mainly static birds the D500 is over kill IMO.
Panasonic G80, 100-400