Best zoom lens for a newbie?

I recently bought a d750 a few months ago and have had a blast with it. I have a few lenses, including an AF-s 20mm wide angle and an AF 17-50mm. Been photographing nature and wildlife mostly, but would love a lens that lets me zoom in much closer than my current 17-50mm to get those up close wildlife shots.

It doesn’t have to be the best on the market, just looking for something with reasonably good quality that won’t totally break the bank. Any suggestions?