I agree with Armeen's recommendation: a C# web api and a light-weight platform-independent UI.
The Web API will be your long term investment, and you can reuse it from now on. With the new 2019R2 scaffolding feature, you will create the API in virtually zero time, and using a technology that you know perfectly (based on .Net DataStore).
Once you have the Web API, you are free to create whatever front-end suits your needs. We usually recommend (with PB Open) a React/Redux front-end, but basically you can choose whatever you like (we follow in this sense customers' preference). If, as Armeen suggests, you use a responsive HTML-based client, you will be able to run this application everywhere, with no deployment. If you will need one day a Native application (that is much more expensive to build, but that is also more feature-rich), you will still be using your Web API.
Good luck with your new app!