I think this is a very very beautiful and thoughtful gift that you are creating for your mother. Mine was sick for three years, and sadly passed away , she was an artist, and also found drawing theraputic!
Im an illustrator, and currently working ALSO on an adult coloring book, so i understand the project at hand and what you would like created for her. Id happily be able to create one with larger coloring spaces so that she can color to her hearts content! have a look through my portfolio, and you will be able to see the quality of my work. ( im illustrating an adult execution book so please dont be alarmed by groosomeness of the themes, just the quality of the work) .
we could actually create an entire PERSONALIZED book, where you could inform me of her favourite hobbies, food types, the best holiday she went on, her favourite flowers, it could be good for her to have something personal and close to home.
The time taken is just a reference but i could do it in less time. I understand you might be looking for someone who charges less, but i garuntee you will not be dissapointed with the quality of the work and how much i invest in my projects and clients. Good luck regardless, i wish you and your mother well:)