There's a lot going on here and I think you've aimed everything at your MIL.
feel like im going mad… im drowning over here. Its non stop.
You do need help. From your husband and your elder two (they didn't ask to co parent little boys at a stage where they're more interested in the world outside, but they're just as much part of the family as anyone else and benefit from mum and dad's labour).
Is there a FIL? Your own parents? Or is she the only one? That's a lot for her, with seven grandchildren. The smaller set up with two probably feels more manageable.
dh works “long hours” 🙄 and when he is home hes knackered so its all on me.
Your “long hours” 🙄 is doing a lot of implications here.
Do you believe he's at work when he says he is?
Do you think he purposely chooses to be absent from the chaos, for the relative peace and quiet at work?
If he's knackered working long hours to support seven, is he actually ok? How old is he? How much of this can he take?
screaming, ds4 having a meltdown cos his uniform itched and ds3 shouting on xbox, and ds1 refusing to come with me cos hes “busy revising” (aka on tiktok 🙄) ... hes moody and just sits on his vape in his room, ds2 is same just glued to his xbox.
Can you not see how she might not feel up to this?
she said she couldnt cos she was “tired” after helping sil all day... never the baby, she always says hes too much.
We don't know how old she is - a MIL/GM of a 16yo could be anything from 50s-80+ and she's almost certainly impacted by peri/menopause.
And putting "tired" in inverted commas suggests she's making this up. Expecting compassion while not giving any isn't fair.