Oh RLC, I do feel for you. I found it much much easier to be a SM before I had my DCs. A lot of that was because, like you, I had been a SC so my sympathies were very much with DSS. I think it is incredibly hard when you have a baby, not least because to DH and all the inlaws s/he is a second one whereas to you s/he is the first. Plus the DH obviously needs to ensure that DSCs feel okay and have as much attention as before etc etc. It's difficult.
I think it can really help to be absolutely consistent about access weekends. This is important because it gives you, the SM, control over your time again. If you are in the habit of changing weekends, it means having to negotiate with the exW and it also means accommodating her if she wants to change. It is a nightmare. After many years of constant changes, we now have a very clear EOW arrangement with flexibility over the hols. ExW was hugely resentful at first, but has now come to terms with a regular arrangement. It is better for all of us (especially my DCs and DSS).
Also, you may need some routines etc to change now that you are a mother. When you are a SM you tend to go with what the DH/DSCs are used to. When you are a mother, you want to put your own systems in place and sometimes those clash. My DSS, for example, never had a set bedtime here. But when I had two small children I couldn't stand him being up late at night - I needed that time with DH, so I instituted one. He and DH didn't like it, tbh, but now it is part of our routine. So much better for me to know that I have a bit of evening time - it makes me less resentful - and actually I think DSS needed the structure.
It's really difficult to just absent yourself from your own home, and as you say, you won't be able to do that when you have a baby. I think once you have two children you have to drop the obsession with special time - it is just to hard to carve up time like that in a big family.
DSS has supper with me and DH on the first night and enjoys telling us about his week. The rest of the time we behave as though he is here all the time and just get on with it. Our kids have also found a way to play together (with all credit to my DSS) so they can amuse themselves as a group. Your age difference is big, but involving your DSCs with the DCs in any way you can is a good idea.