The D7500... or D500... or, for that matter, D7200 would be your best bet.
All three are DX cameras. Those give you "extra reach" over an FX camera like the D750.
IF you were mostly interested in portraiture (people, not birds)... or landscapes... or architecture... maybe even macro... the 24MP D750 FX would be a fine choice. It requires FX lenses, to get the best out of it (can be used with DX lenses, but the cropped images will be under 10MP).
But for birding... sports... wildlife...the 21MP D500 DX is a top choice. The 21MP D7500 DX is close behind, sort of a scaled down version of the D500. The 24MP D7200 DX is very good, too... higher resolution than the other two (same resolution as D750), still fast tho just a little slower... and is very seriously discounted right now.
All four would be a nice upgrade from a D3000-series or D5000-series DX camera. All four have a true pentaprism (instead of a penta-mirror like the D3000/5000 use), which makes for a bigger, brighter 100% viewfinders. All four have the in-body focusing motor that allows older Nikkor AF lenses (and similar) to be used. Those lenses are unable to autofocus on D3000/D5000-series cameras, which don't have the in-body focusing motor. Those cameras can only autofocus AF-S and AF-P lenses (the latter requires more recent model D3000/D5000-series). All four cameras use the EN-EL15 type battery. All four can optionally be fitted with a grip for added battery capacity and vertical controls.
Key points of difference....
D750 $1297 (also seriously discounted right now... $700 off)
- 24MP FX
- EXPEED 4 processor
- 51-point AF with 15 "cross type" points
- top shutter speed 1/4000 (1/200 flash sync)
- 6.5 frames per second continuous shooting rate
- ISO 100-12800 (expandable)
- tiltable 3.2" 1.2 million pixel rear LCD
- HD video up to 60 fps
- dual SD memory card slots
- WiFi
- 1.65 lb.
D7200 $697 ($400 off regular price)
- 24MP DX
- EXPEED 4 processor
- 51-point AF with 15 "cross type" points
- top shutter speed 1/8000 (1/250 flash sync)
- 6 frames per second continuous shooting rate
- ISO 100-25600 (expandable)
- tiltable 3.2" 1.2 million pixel rear LCD
- HD video up to 60 fps
- dual SD memory card slots
- WiFi
- 1.49 lb.
D7500 $1147 ($100 off regular price)
- 21MP DX
- EXPEED 5 processor
- 51-point AF with 15 "cross type" points
- top shutter speed 1/8000 (1/250 flash sync)
- 8 frames per second continuous shooting rate
- ISO 100-51200 (expandable)
- tiltable 3.2" 922K pixel Touchscreen LCD
- 4K video up to 30 fps
- single SD memory card slot
- WiFi
- 1.41 lb.
D500 $1797 ($100 off regular price)
- 21MP DX
- EXPEED 5 processor
- 153-point AF with 99 "cross type" points
- top shutter speed 1/8000 (1/250 flash sync)
- 10 frames per second continuous shooting rate
- ISO 100-51200 (expandable)
- tiltable 3.2" 2.5 million pixel Touchscreen LCD
- 4K video up to 30 fps
- dual SD memory card slots
- WiFi
- 1.89 lb.
https://www.bhphotovideo.com/c/compare/Nikon_D7200_vs_Nikon_D500_vs_Nikon_D7500_vs_Nikon_D750/BHitems/1127271-REG_1214161-REG_1333060-REG_1082599-REGThe D7500... or D500... or, for that matter, D7200... (