Completely understand where you are coming from OP. My MIL was like this - wanting to clean, mother and "help" with our day to day lives. Well-intentioned,but it just caused resentment and frustration with both DH & I. Just as some examples -
She would buy us things we didn't need or wouldn't use . eg. new toilet cleaning brush, when nothing was wrong with the old one – it's for cleaning the toiletfor christ's sake!, numerous tinned veges or beetroot or canned fish which we don't eat, meanwhile she would be sooking about how she was struggling for money (um ok, so stop wasting money buying us stuff we didn't ask for or need!)
Doing jobs in a way that we'd have to re-do, or that didn't need doing, or causing problems (eg. snuck in & took our dirty washing home with her to wash ("tosave your water!"), and then she kept mixing DH's clothes up and his brother ended up with them.
Not taking "No" for an answer and doing things anyway– once our dogs got fed 3 x times normal food in one day because she ignored DH telling her 3 times viatext that we would feed them when we got home, she didn't tell us she'd fed them and so we thought they were all sick because our usual garbage guts dogs wouldn't (couldn't!) eat dinner we'd gave them.
House work, I felt the same as you – it's a personal space thing, and I like to do things my way.
This may not work for you, but seeing as a polite "thank you, but we'd rather you not do that" just got us a "why can't you just be grateful" and a side fisheye, I went a bit of a "Kill her with kindness" approach –
"Oh no, no, don't you dare touch that washing up, I would feel bad you doing that when you've got to go home to cook dinner for you & FIL and do your own washingup, I couldn't possibly let you do that!!"
"OH MY GOODNESS, I would be so embarrassed for my husband's mother to wash my underwear, you can't see that!!!" with a very embarrassed expression that impliedthere was some very scanty things in there.
"It's getting late, I would feel so bad you driving home late in the dark because you stayed to help me, you put that down now and head home so you avoid thewildlife that's out on dusk, it's not safe!"
"You've been so busy yourself lately, you must have your own housework to do and I would feel so guilty if you were here doing mine!"
I would just keep repeating things like this with a smile on my face.
This has worked for me – it's still a bit of work but as she won't respect healthy boundaries or the word "no", this is a least drama.
I'm slightly different situation in that MIL frustrates DH (her own son) just as much being like this, he has no trouble being blunt with her and doesn't careif she takes offence or sooks to the rest of the family about it. I just had to take a different approach to keep a bit of family peace as I'm a black sheep DIL.
Could you use the same approach but sell it like its for baby's benefit? "Now you relax and enjoy your time with darling grandson, I will feel so guilty ifyou do any housework when you are supposed to be bonding with DGS, I will sort that housework out when I get home, don't you dare lift a finger!".