Like all of us he is not perfect, but 99.99999% of the time he is not an arsehole.
Why would you post if this was the case? I don't want to make your problems seem worse but even if it is a tiny amount of arseholiness then it would not preoccupy enough of your thoughts for you to bother writing this post. I suspect his behaviour really bothers you a whole lot and takes up a reasonable percentage of your thoughts... so really even if he is 0.00001per cent arsehole, it has a larger effect on you. It's about what it says about your relationship, and this....
I have to think and plan really carefully about how to broach difficult stuff with him To try and avoid this reaction and allow an adult conversation. I find this quite draining and as a result am less likely to tackle stuff that should probably be tackled in a healthy relationship.
... is a description of what is commonly known as 'walking on eggshells'. The fact that you are walking on eggshells is an indicator that he may be anxious, but he is also controlling. And he is wont to control you. This is the deeper fear in your OP that goes beyond the 0.0001% or whatever quantity you define.
And your question 'how do I deal with dp's defensiveness?' is, I'm afraid a sign that you think you can change his behaviour by being more diplomatic, better at explaining, more articulate, more sensitive or thoughtful or that there is some magic way of relating to him that will make him nicer to you. OP - truly, it is not that you are lacking in some way. It is not that you are explaining things badly and putting his back up. It is not that you are inconsiderate or nagging. It is that he is putting you in your place. And it is working for him as you attest, since you are (repeating, I know) having to think and plan really carefully about how to broach difficult stuff with him To try and avoid this reaction and allow an adult conversation. You are being reasonable. He is being extremely UNreasonable.
It is understandable that you are paddling back from your OP in your subsequent posts, because people are telling you he is not vulnerable but in fact quite controlling and deliberate. This is not a pleasant thing to hear.
So, it really doesn't matter about the walking or when you do it, what really matters is his behaviour towards you and whether his 'defensiveness' is actually just nasty and manipulative behaviour to get you to tiptoe round him and to very effectively shut you up.