You are lovely and your in-laws are incredibly fortunate.
Your situation would drive me insane, whether it was MIL, my own mum or dad or even my best friend.
Every other day would be too much for me too but everyone is different.
Your sweetness means you have actually been reinforcing her bad habit. Her calling in unannounced has been rewarded more often than not. I think it will improve when you stop that. You don’t have to be horrible, but if she knocks and you are in another room, as previously, just don’t respond. If she has peered through and seen you so that’s not an option, go to the door and open it a bit and say “Sorry Celia, I missed your call, it’s not a good time now, we’ll catch up on X”, and say goodbye and close the door!
She really is pushing it, you should have no qualms. If she gets upset and says she feels like she has to make an appointment, don’t backtrack, point out that you have repeatedly asked her to call ahead and she has seemed to agree but then just does the same thing. To you, that’s not making an appointment, but if to her it is, then yes, can she please do that as you love her dearly, but it’s not normal in your culture and you find it stressful.
Your softly, softly approach hasn’t worked, you need to enforce your own boundaries. She won’t like it, no matter how kindly you do it, because she wants to be able to drop in without warning on a daily basis, sometimes even more than once. You, very reasonably, don’t want that.
As you have a great relationship otherwise, she will get over it, you just have to brave her initial upset.