I didn't have the conversation, as the doctor rarely saw my mum or dad, who remained in physical good health, despite severe dementia - you are fortunate having a medical person who is walking this path with you. Do let yourself be guided by them. (Is your mum at home?)
I would also say trust your instinct to focus on comfort, not life lengthening treatment, and spend as much time as you can with her, just holding her hand, and taking little steps to keep her comfortable - vaseline for her lips for example, or sips of juice to keep her mouth moist. It is gruelling, but I found afterwards it was a reassurance to me that I did as much as I could for her. And in moments of lucidity, she was able to recognise me occasionally and manage a smile.
I'm coming to the conclusion that there is no easy way out of this world, for the person leaving, or for those who love them. It's so hard to know what to do - we get all of that support before giving birth, but there is so little support in helping someone to die. I did have "With the End in Mind: Dying, Death and Wisdom in an Age of Denial" by Kathryn Mannix recommended, but never found the emotional space to read it. It might help.
Like your mother, my Mum found any intervention distressing. (One of her remaining words was "No" so she used it liberally. She just wanted them to stop doing things to her - even things that would make her more comfy.)
I did have a conversation with a very supportive nurse who reassured me that it was not unkind to refuse further treatment - quite the opposite - and with that in mind, as I care for Dad, who also has dementia, I will not be pushing for many treatments for him.
Hope that helps.