Try this

```lisp
(create-fontset-from-fontset-spec
 (font-xlfd-name
 (font-spec
 :registry "fontset-mine"
 :family "SF Pro"
 :weight 'regular
 :slant 'normal
 :width 'normal)))

(defface my-face nil "My Face.")

(set-face-attribute 'my-face nil
 :font "fontset-mine"
 :height 1.0)

(add-hook 'org-mode-hook
 (defun my-org-mode-hook ()
 (buffer-face-set 'my-face)
 (face-remap-add-relative 'my-face
 :height 1.5)))
```

Instead of rolling out your own `my-face`, you can as well work with `variable-pitch` face. IOW, you can get the same effect by

- setting the `:font` property on `variable-pitch` face
- put the `org` buffer in `variable-pitch-mode` 
- do a `face-remap-add-relative` as above

Before putting `org` buffers in `variable-pitch-mode` I suggest that you look at prior work like this one [Ricing up Org Mode](https://lepisma.xyz/2017/10/28/ricing-org-mode/)