You've been married to this younger man since 2020, you have two adult children. Can I ask - did your husband move in with you, to your already-established home, or did you both find a joint house to move into together? Because "He also had a moan today and has said that after Christmas, if my son doesn't contribute to the household financially, then he isn't going to either" strongly suggests to me that your husband moved into your already-established home.
The reason I'm asking is, your husband could be feeling like more of a lodger than a husband if he moved in to your home. He lives there, but has no say in how the household runs. Has no say in who lives there, has no say in who visits and how often, cannot control his working environment, has no say in the overall finances. I'm not saying he should have a say in all this, but I am pointing out that you calling all the shots could leave him feeling like a lodger. Insecure.
Your adult children are presenting you with two separate issues.
Your son - "I've always told my kids that whilst they are in education, I would support them financially, or at least not take housekeeping money from them." and he goes back to uni in January. That's a pre-existing agreement between you and your son, to take no financial contribution from him. Quite common, I did the same with my son. From your husband's point of view - how much longer will your son be living with you both? When can he expect to you just living as a couple?
At 23, is your son doing a Masters, or did he take a couple of years out? Is there anything in the set-up that might make your husband feel that your son is milking it? I do feel that this is resolvable through a bit of conversation and managing of expectations.
It's also possible that your husband might just be kicking off about your son because he's 'kitchen sinking' - your son is a small problem but he's overwhelmed by the big problem of your daughter, so drags in this smaller problem to convince you that something needs to change.
And so to your daughter.
"... she just turns up saying "I'm knackered" or something similar, so I feel bad and make them breakfast or lunch (depending what time they turn up) and try to help out with dgs. I try to contain him to the living room whenever possible".
From your husband's point of view, your daughter "comes round "shits the house up and just leaves" and let's my grandson "scream his head off" whilst he's trying to work". You also mention slamming doors. These are legitimate complaints on your husband's part.
You do this for your daughter because you believe it's helping her. Have you considered that in the bigger picture - it might not be helpful at all? That you are instead facilitating her partner's continued crapness by picking up his slack? That you are effectively making it bearable for her to stay in a crap relationship, rather than supporting her to either sort out or to exit her crap relationship? Instead, she comes to mummy; has a moan; has mum feed her, watch her son, clean up afterwards - and goes back to exactly the same situation she left that morning. And then starts on the same hamster wheel the next day. Big picture - not in her best interests, is it? And frankly, not in your best interests either. It's not just the menopause leaving you short of emotional energy - your daughter is draining you.
"I just feel like I'm trying to do my best by my dd as she's very depressed and overwhelmed and has little support from dgs's father. I don't want her doing anything stupid, she's spoken of suicide before and I couldn't bear it."
You are trying to do your best, but it's draining you. And once you are fully drained, you won't be able to do anything to help her. Not a thing. So please, prioritise YOU. Use the analogy of an aircraft in bumpy weather. The oxygen masks come down, the instructions are to fit your own mask before fitting your children's masks on. Because you're damned all use to them if your unconscious from lack of oxygen. Your health, your wellbeing - you need to prioritise these.
You need to explain to your daughter the effect she is having on YOU. That you need to change the dynamic. When she comes round - can you three go out? To the park, a soft play (doesn't sound as if a cafe would work with your DGS) or just back to HER house? It's unfair for her to come and affect your husband's workspace and to prevent you from working too. And if she's depressed, you are not going to be able to affect that. You need to point her to her GP, therapy - professional help. I find myself wondering if she spoke of suicide to keep you on her leash. Because you are putting your own life on hold, just to keep her on her hamster wheel. And it's not working for either of you.
((hug))