What can I do to make this work properly?
At the moment, I don't know of any vertical resizing, but horizontal resizing in symbols can work with grouped text elements. Usually, it works by pinning the text areas to the desired side (right or left). With this done, and a bit of testing, stretching the symbol should allow you to add text.
Vertical text resizing would be a great future addition, though.
First, you need to install plugin called Anima (https://www.animaapp.com/)
Second, you need to watch a gif :)
Maybe this will help you with that quest: https://medium.com/sketch-app-sources/overrides-dynamic-buttons-in-sketch-c9ccfb6a5682
It is a plugin that helps buttons have a dynamic button widths. So the style via a symbol stays, but the shape of it changes. It may also work for vertical content.
Also the new and improved resize tool can help you with this job. More info abou tat one you can find at https://www.sketchapp.com/learn/documentation/layer-basics/constraints/