I think you should be careful of “perfect is the enemy of the good”.

I’m sure a mobile app would be the perfect solution. Nevertheless, until that materializes, it would be worth investing a day or two to mobile CSS.

It won’t be as good, but for people wanting to use your product now, it will be sufficient, and will be the difference between them using it and not.

There is a saying in the graphics industry, "Output drives process." Always design with the output in mind. As this is a project that took years to create I can see how designing with responsive in mind could be overlooked or put on the back-burner. That being said, it will be a thorny bush to prune doing it later rather than sooner. Good luck.