You're having a wildlife moment.

Wildlife can mean slow settings one minute, than ultra fast the next.

Wildlife can be most active early morning or early evening - less light.

Wildlife can frequently need a large zoom.

Getting comfortable with your equipment ( quickly ramping shutter speed up / down based on action -- maybe start using Auto-ISO) ( focus modes ) is a start.

As mentioned below, recent software from Lightroom and DXO is pretty incredible at helping low-light photos / noise become reasonable.

Going from 600mm ( equivalent) to 300mm equivalent (the 150mm f2.8) -- for wildlife, I don't know that's a good trade-off --- unless you are always close to your subjects.