My next prime portrait lens should be ... ?

theninth wrote:

...

Regarding focal length, I would not go any longer. 85mm is just right for headshots with pleasing proportions

If by "headshots", you actually mean headshots, which is to say, framing tighter than "head and shoulders", then I'd have to strongly disagree. On a 20cm x 25cm print, (the typical "8 by 10 glossy") a head shot frames an area on the subject plane of about 24 cm x 30cm, cropped from a 3:2 aspect ratio frame that covered 24cm x 36cm on the subject plane.

To frame 24cm x 36cm with an 85mm lens on a FF camera, you'd need to be 0.9 meters  (3 feet) from the subject. There is no way such a subject distance is going to provide  pleasing proportions.  You are going to get comic or tragic distortion.  ID shots, which are not known for their pleasing proportions, are typically taken at around 1.8-2m (6'-7'). To get pleasing proportions of most adult subjects, you should be at least 2.5m away and 3m to 5m is better.

One of the most common mistakes made by uniformed amateurs it to think that, since an 85mm lens is called a "portrait lens", they can successfully use it for all sorts of portrait framings. It is just way too short for pleasing head shots. It is too short for good head & shoulder shots. On FF, it's best use is for 3/4 height and wider, though it can be used for half height in a pinch.

and still wide enough for full body shots with a huge amount of background separation. Yes, 105mm and 135mm can do that as well if you step away even further, but I think the compression is too much then.

Did you really find this shot too compressed?

And if talking 85mm, the new RF 85/1.2 is simply the best out there, so getting it ensures to silence any future nagging what-if voices in your head.