OP, so pleased to see your updates this evening. Knowing that people believe you and are going to help must feel like a big weight has been lifted from your shoulders. The table and dinner tonight sounds fab, I wonder if DS and DD were receptive because you and DP showed a united front? If you are both consistent it will make things so much easier.
Do speak to the GP about depression still. As a PP has said, there is such a thing as reactive depression, where you are depressed by your situation. A large number of SEN parents I know take antidepressants to help them deal with things, some of them long term. I found CBT and mindfulness helps me more, but it's very individual finding what works for you and you must feel no shame in taking all the help YOU need for you personally. The GP may want to run through the depression scale with you, you might have done this at your postnatal checks. It will give them an idea if you need support in that area.
If DS is diagnosed with anything, get yourself registered with Carers UK and sign up to your local branch too. You will be able to get respite or grants to help either your DC, or to use towards activities and respite for yourself. Some I know get gym memberships, others take adult education courses etc. Looking after yourself will be a big part of keeping you calm to parent effectively.
I know this morning didn't start so we'll although the day has ended on a high for you. Just remember that sometimes it might feel like one step forward and two (or more) steps back. That is TOTALLY NORMAL. Don't lose faith. There will be bad days where he might try to test if you're sticking to new rules. Show him you are and it will improve again. I've been there, at the "Why is he doing this again after he stopped?!" stage. I said to my DM yesterday that I still learn new ways to manage DS weekly, sometimes even daily, and he's 12. I think your threads have made me stop and think about my parenting in the last few days, not in a self congratulatory way, but by considering what I could still do better! 