It's not really about the chocolates is it?
My GP told me once the most important sentence in a consultation isn't what the patient says when they come in and sit down, it's what they say when they have their hand on the door handle at the end. I think it's maybe about your last sentence in your post. "I may be a bit sensitive as it has been touch and go as to whether I would even still be here now hence why I am indebted to this consultant."
Perhaps there is a little part of you that feels a little let down and perhaps resentful that your specialist is "moving on". Maybe you're also nervous about your new consultant? I don't think you really give two fucks that you haven't had a thank you letter
but perhaps you do give two fucks that feel like you're being a bit "abandoned" by someone who you trust, and are very grateful to, and feel like you rely on.
Very very understandable from anyone that has been in that situation. No doubt though you will get a load of dicks not RTFT and jumping in to tell you how U you are being 
I understand where you are coming from, but you have done a kind thing in sending the chocolates and note. You're not in the wrong here at all, and your feelings are very reasonable, I think, under the circumstances.
Also, just to say when I was in that situation (after taking it very personally that my consultant was leaving me
) my new one was wonderful and actually having a fresh pair of eyes really helped my care. I hope it's the same for you.