All this is just my thoughts ofcourse. So...

You can do a "soft startup" to avoid hurting her feelings. But IMO do not do that at the expense of expressing what your issue is. Sortof more like providing the message inside a "s**t sandwich" instead of being all nice and pleasing or placating in your entire message.

If she hears you but then she doesn't respect your request afterwards or plain forgets to keep it in mind consistently enough, you can introduce natural and reasonable consequences for it. Nothing like threatening to the overall relationship though, just simply to reinforce your boundaries around these things.

Like, if she calls at a time that she's been told is not okay for you, just do not take the call. Since at this point you've already informed her that it's not okay and that you are not going to be able to take such calls, and that you are only able to take calls at x or y times. She will then learn from consequences if she forgets this message at first.

If she is unwilling to try and hear you in the first place, I would simply take a break from the relationship for a long enough time (only you know what's long enough for you), without discussing it further, let alone asking for her permission for it. Informing her in another "s**t sandwich" that you are not going to be available for a few months.

Then when you feel ready later, you can try discussing the problem afterwards when you both are in a good place to talk it over.