You were concerned about the VR not being enough on the 500 pf. What shutter speeds could you use it at successfully? On the D850 and Z9 I could get 1/100sec with almost 100% blur free - always dependent on the subject being still! Again, like Richard and JCGamble have stated, length, length and length!
Just so as you can rest assured that the 500 + 1.4x TCIII combo is good on a Z9:
All these birds have a body about the size of a man's thumb! They are tiny and you need the length!
Beautiful Firetail:
Z9 + 500 f5.6 pf + 1.4x TCIII, 1/320s f/8.0 at 700.0mm iso900
Z9 + 500 f5.6 pf + 1.4x TCIII, 1/320s f/8.0 at 700.0mm iso900
Variegated Fairywren:
Z9 + 500 f5.6 pf + 1.4x TCIII, 1/400s f/8.0 at 700.0mm iso125
Spotted Pardalote:
Z9 + 500 f5.6 pf + 1.4x TCIII, 1/800s f/8.0 at 700.0mm iso1400
Southern Emuwren
Z9 + 500 f5.6 pf + 1.4x TCIII, 1/400s f/8.0 at 700.0mm iso6400
Zebra Finch
Z9 + 500 f5.6 pf + 1.4x TCIII, 1/500s f/8.0 at 700.0mm iso1100
Female Golden Whistler
Z9 + 500 f5.6 pf + 1.4x TCIII, 1/400s f/8.0 at 700.0mm iso3200
These are Pied Oystercatchers but this is as close as they would let me get and this is about a 20% crop
Z9 + 500 f5.6 pf + 1.4x TCIII, 1/640s f/10.0 at 700.0mm iso250