Yep! Sorry if this sounds rude op, but I question the intellect, or at least the common sense, of these over-bearing mothers, who attempt to control their relationships with their adult dc through fear, obligation and guilt because it always has exactly the the opposite result to the one they are aiming at!
If you want your adult children and your grandchildren to engage with you, then you take the pressure off, ensure your interactions with them are enjoyable and guilt-free, and accept the fact that they are the star players now! It also helps if you love them unselfishly and you can be genuinely happy for them and the independent life they have created because it means you have done your job as a parent well!
You recognise that their young family comes first now and your job as grandparents is to live your own interesting independent and fulfilling life too, but be on hand to interact with adult children and grandchildren when the opportunity or need arises! Step back and get out of your own way in other words! And everyone else’s!
In your shoes op, I think you need to have a calm, honest face-to-face word with your mum, and yes, tell her that her behaviour makes you want to disappear, even though that’s hard for her to hear, and that from now on you will be seeing her the first Saturday of every month, or whatever schedule you feel comfortable with, take it or leave it, because your patience is wearing thin.
It’s time to take the initiative op and set some solid boundaries. You don’t need permission. You are your own person with your own limits. A boundary is about what is tolerable for you and no one else.
Tell her that she needs to find fulfilment in her life in ways that don’t always involve you. Stay strong op. Your mum has issues that are really far beyond anything you can solve, and probably always will, so every time she kicks back, which she will, suggest the name of a therapist in her area who has experience of treating an older age group. She probably won’t go to therapy but it will make the point that she has options other than engaging with you and making your life unbearable!
All of this will seem very strange and difficult at first and you will feel guilty but you have to stick to your guns so that your mum knows you are serious. Good luck 💐
Edited to say that: I agree with pp, who wrote that you will find a different kind of strength when your own children are involved. In my situation, the overbearing family member wasn’t the mother of me or my dh, but we tolerated their behaviour when it was just us they were trying to manipulate, even though it caused us considerable stress, because we had been brought up to be respectful of our elders, loyal, and to try and always work through family problems in a reasonable way. None of this worked of course but when this person tried to turn their manipulation tactics on to our young dc, we had no compunction about putting in very strong boundaries.