Sounds like you did. Shows how much of an absolutely amazing person and mother you are that you got through that and have a great relationship with your son now. Even though everything was so hard, you cared so well for him that he developed in every way including building his trust and love for his Mum, knowing he was secure and safe and cared for. You did all the right things, you just weren't getting much enjoyment out of it - but he didn't know that because you are such a good mother.
If the same thing was to happen again I am sure you would recognise the symptoms. If you never have another baby and your DS doesn't have a sibling that's ok too. My DS is an only and he sees advantages and disadvantages to it. He has friends and cousins and he has friends who are onlies and friends with siblings and he's happy. Sometimes you have to do what's best for the family you have already, though I would have loved another child I'm a bit too old now and DS is 11 so they wouldn't be playmates anyway.
Sorry for the ramble! I think you're great and your DS is lucky to have you, your love shines out. We can't change the past and you have nothing to feel guilty about. You did your best and your best was more than good enough. You've built a fantastic bond and you did it through difficult times.
I had a terrible bereavement a few years ago. Up until then my DS was the sunshine in my life and I'd get a great rush of love when I saw him, when he woke up in the morning, when I picked him up from school, when I watched him playing or sleeping or whatever. But for a good 18 months I had no joy, I was just going through the motions. I know he felt it a bit and I do feel guilty, but he's ok and he was ok and I was still there loving him and being his mother - it just wasn't as joyful or enjoyable for me because really I just wanted to curl up in a ball and howl most of the time, especially at the beginning... but I was still there being Mammy and cuddling him and listening and playing and reading stories. Santa came, birthdays were celebrated, homework was done, dinners were made. So for him I don't think it made much difference, it was more for me not feeling the joy or connection. And I lost that time too, it's not coming back.
And that's life. There are ups and downs. And you've already proved you are a good mother during the downs. I do think you might need to grieve a bit about missing out on that baby time. Grieve and feel sad and forgive yourself and let the guilt go.
I think if you were to have another, you may not get PND anyway and if you did you'd recognise the symptoms. So follow your heart. Best of luck and sorry for the big ramble. I'm not sure it's at all helpful - just wanted to know you're great and not to feel guilty and life can be hard at times but that just shows how great you are.