Totally agree that the 1.8 is a steal, but with a $1500 budget, you've definitely got room to get something with a bit more *wow* factor. Since you're shooting headshots and want that creaminess, the extra aperture or focal length really helps. Tbh, the Zf looks so good that you kinda wanna match it with some glass that has real character. I've been looking into the rendering specs lately and these would be my top picks for your budget: * Nikon AF-S NIKKOR 105mm f/1.4E ED - You'll need an FTZ II adapter, but man, the bokeh falloff is just unreal. It's basically a legend for headshots. You can usually find them used for around $1,100-$1,300 these days.
* Nikkor Z 105mm f/2.8 MC S - I know it's technically a macro lens, but the compression is fantastic for tight portraits and it's super sharp. Usually runs about $1,050.
* Voigtlander Nokton 50mm f/1.0 Aspherical - If you don't mind manual focus, this looks amazing on the Zf and the f/1.0 speed is just insane for bokeh. The 105mm 1.4 is probably the winner for that 'creamy' vibe you're after, tho it is kinda heavy. I'm not 100% sure how the balance will feel on the Zf without a grip, but the results are gonna be stunning regardless.

I've used the Nikkor 85mm f/1.8 S for years. Stunning portraits, and a bargain at around $550.