Honestly, it sounds like they're doing September. It's about knowing what you can control, and what you can't, and if they've decided they will write in September, you'll get a letter in September. I know that's harsh, but being upset over that isn't going to help you keep contact with your son, and I know you are good at being pragmatic. So, in terms of your options, you can say 'you can write in September, but I cannot view it until October', which the parents may be ok with, and then they can expect a reply in November.
I would leave the future to the future- they may or may not involve your son in contact in the future, if that happens, things can be looked at again.
As you said above, it sounds like they've decided September, so it's working with that. You can't view the photo until October, which is a delay, but if contact is taking place every 6 months, it's not like you need to reply by first class post anyway. If you book to go in Monday of half term, you can get the reply sent a week later, meaning your reply is still there before Christmas madness. TBH, your reply may well just sit in a SS office for a week or so, anyway.
We changed our letterbox months without running it past the BPs, but that's because they haven't engaged at all. I also wouldn't reply to SWs outside contact now, we want to be free of them.