When I saw your question, I felt that your mother is similar to my mother. So I will talk about my thoughts based on my own experience. Last year, on my mother's birthday, I went home to help her celebrate. During dinner, I asked my father what gift he would give her. My father said that we are an old married couple, so why should we bother with that? At this time, I clearly saw a little lonely look on my mother's face. Later, my mother said that she was actually looking forward to my father's gift. So, here, my suggestion is that you can buy a gift and give it to your father, and let your father give it to your mother.
For example, buy a bunch of roses, because in our parents' generation, they were not so romantic. I think that in that case, my mother will definitely be very happy, which is happier than receiving your gift.
You can also buy a cake and go home to celebrate your mother's birthday with your parents. Having a husband and a daughter is the best gift. What a mother wants most is a harmonious and happy family.