OP this really doesn't sound very good at all. What you are describing suggests not a normal, balanced dynamic in your relationship. Perhaps you need to hear this:
In an equal, healthy relationship, both partners take joint responsibility for the running of their combined lives, including such things as life admin, and remembering the things that need to be done in order to maintain a home, a family and a social life.
In an equal, healthy relationship, difficult discussions can be had without anger, and compromise can usually be found.
So your combined posts sound concerning and I suspect are the tip of the iceberg:
He doesn’t work, he’s in his fifties, he likes to sit down all day. That’s the situation. I work school hours and evenings. The weekends are days off for me to do the jobs that need doing.
Why doesn't he work? What contribution to the running of the household and family does he make while you are working? What are the jobs you are doing at weekends, and what are the jobs that he is doing at the weekends?
he doesn’t see the point of housework, doesn’t like the “mundane repetitiveness” of school runs and hates organising bills. I last brought up dividing housework jobs about a year ago and he was quite angry quite quickly that I was wasting time talking about it. I really hate the mundane repetitiveness of some of life's chores, but I don't get to simply opt out. Neither do you. So why does he? What did he do before your relationship, live in a hotel?
Sticking up for myself is a new thing for me and I don’t let things escalate into arguments. If he says no, I’ll accept it and not grumble From your comment above about him getting angry if you raise something you don't like, and this, it sounds like you avoid conflict and agree to things you don't like so he doesn't get angry at you. Does that sound right? Why does he get the final veto on things, instead of you?
We’ve both been working on “making a positive request” so it’s not a nag, a demand or anything else negative. Supposed to say what you’d like help with and what difference it would make and how it would make you feel… Whose idea was this? What sort of things does he ask for your help with? It may be a stretch but given everything else it sounds like this comes from a fundamental issue with division of responsibilities in the first place - he sits on his backside all day, you do everything, and now you're learning how to ask him more nicely to help you with doing his share???
Sorry OP. Some of the above may be fair or unfair, but it doesn't sound great from what you've written. Agree with PP to start a thread in relationships for more nuanced advice.
Ps hard agree lemon icing on Christmas cake is a no 