You sound very kind and mindful of your relationship with your MIL.
In your shoes, I’d probably approach it this way.
Start with appreciation:
"Hi [Mother-in-Law’s Name], I really appreciate you helping out with [Baby's Name] two days a week—it means so much to us. We know you have a lot of experience with feeding babies, and we trust you, but we wanted to share some things we’re focusing on right now with [Baby's Name] and her nutrition."
Provide gentle information about nutrition:
"As she’s getting older, we’re starting to introduce more variety to her meals to make sure she’s getting all the nutrients she needs. Right now, she’s ready for more protein and some different textures, so I’d love for you to start adding some soft, mashed foods like [suggest some protein-rich foods like scrambled eggs, lentils, tofu, or chicken], and maybe some mashed veggies like sweet potatoes, peas, or carrots.
We are happy to pack a few things for the next days you are taking care of her, if you want to know/see how we normally do it at home?
Most mashable foods, cooked without salt, that you normally eat for your tea/dinner, can be mashed up and given to her.”
Offer support and flexibility:
"I know that she’s already loving the porridge/rusks, so we can still keep that as a small part of her meals, but we would like to start incorporating more variety when you’re taking care of her. If you ever need any help with new ideas or preparing the meals, I’m happy to help too!"
End on a positive, collaborative note:
"Thanks so much for understanding. We really appreciate all the love and care you give her!"
I think you want to keep her involved, but just give gentle guidance. Good luck with it all.