Ok, you're interested in psychiatry and stuff which is good (you have something, you'd be amazed how many people have nothing).
You should (imo) put something about one of the books which really grabbed you and you found astonishing. Something you want to know more about (the brain and stuff is good for this). Having worked with autistic children you can talk about that and how seeing people with a mental condition who don't think like everyone else (don't mean to offend if i do but, you know, it is true) is something you want to understand. Which I think works well as a vocation
I think this would be a good talking point since you've said you are interested in it and have proof since you've followed that up. You could mention working in an old people's home if you think you have enough to talk about regarding it. Talking about people mentally deteriorating and wanting to help or something could work.
Idk what you did when you shadowed a GP (I've never visited my GP so I honestly have 0 idea what they do) but its worth throwing in there. It is something which will very be relevant even if there isn't much to say about it.
Now, I'd probably try and be more tactful and well written than I have here. However, I think you've got a lot to work with. My advice when it comes to writing it is to dive straight into the meat of your PS almost immediately. Start with a brief comment about medicine (say its unique or something idk) and then dive straight into the book. You can easily steer that into you volunteering.
Alternatively, you could open with your vocation but I think this often sounds forced. I think it'd be better left until later once you've gotten the reader onto your side.
Oh, you should take all of this with a grain of salt btw. I don't work in admissions and just because i got offers doesn't mean I'm good at this. (Heck, I'm a STEM student).