For all interested, here's how I solved it:

1. I defined the "executor" <input> as disabled and readonly
2. I made the "executor" field in the form not required
3. I also put self.object.executor inside form.initial on form render
4. On form_save(), I replace form.instance.executor with values from form.initial

May be hacky, but seems pretty bulletproof to me. And the code is pretty clean!

Thanks to @ru, @bpepple, and @a for helpful tips! ✨