Doodledog
I wonder if she’s struggling to make the transition from mum-child to adult-adult? After so long as the one at the centre of things, providing care, food and pretty much everything for her family she is now a MIL, like a dowager who has to defer to the next generation. That can be difficult for some people- particularly if they made their family their world. She might be reminding you of all she does for you to try to regain some of the maternal status she’s lost.
I know it’s irritating- as I say, I have a similar situation with my mum - but it seems to me that you can decide not to put up with it, and put her in her place, reminding her that she is basically irrelevant and unneeded, or bite your tongue and thank her profusely for what she does, whilst carrying on living your life as you choose.
The first option may be satisfying- you will rid yourself of an irritation and firmly establish yourself as the new centre of things, but it risks badly hurting the woman who loves and brought up your husband to be who he is. The second won’t displace you in any way, but will let your MIL cling to her (imagined) role for a while longer. It might be the less ‘honest’ way, and it isn’t something you have to do at all, but it seems to me by far the kinder opinion.
There is the third option
Thank MIL as you would anyone else, and refuse to play into her neediness.
You are still doing the right thing by thanking her, but you are refusing to give in to her (possibly unintended) manipulation.
My MIL would do this. Offer to do something - refuse all offers of help, and then complain that no one helped. It was maddening until I realised it was just who she was. I offered, shed say no, and wed both go on to do our respective things.