I tend towards either anxiety-induced non-reponse and feeling bad about it or writing something way too long and worrying that I came off weird or that the person will feel obligated to respond (which filtered back through my own experience is tantamount to psychic violence).

Putting on my cognitive behavioral hat, making a late reply is good as a low stakes exposure exercise. You kill two birds with one stone--you show yourself that you can push through avoidance even after it has built up, and (with almost complete certainty) by receiving either no response or a neutral/positive response you can see that no calamity will befall you for daring to necro an old post.