Thank you for sharing, my condolences to you, that is so hard.
As @quirkywords said above, would it be worth trying to find a compromise on the plaque that still includes the spirit of what you want to write? By the sounds of the way you cared and loved your mum, your message and memory of her is more than just that plaque, and you will always know what you shared with your mum.
My partner lost his mum just last year, and I really admire you being as open as you are. Something that helped him was reflecting on what would have been at Mother's Day and her birthday, and planning a commemoration on those dates in smaller ways. Can you celebrate that 'big family party' on your own, in your own special way? Or organise something smaller online? Seeing my partner, it was hard and kicked up a lot of tough emotions, but they were really worthwhile times.
Thinking of you, you are a strong individual who has so much to give. You've got this!