Which combination of programming technologies would you recommend for?
Two scenarios, two solutions. If mobile apps are not of critical importance, go with C# + .NET Core + WPF/UWP + SQL Server. Get all benefits of native development. Host your database in SQL Server, write a lush client application in WPF or UWP using Visual Studio 2019 on top of .NET Core 3. End of story. If mobile apps (iOS, Android) is a must, go with client/server architecture. Host your database (MySQL, SQL Server, whatever) with a thin REST API around it using a server-side framework (Laravel is my favorite). Write native clients for each platform; Windows client in WPF + C# + .NET Core 3, Android client using Java + Android Studio and iOS client in Apples tools. All platforms provide native tools to communicate with servers using HTTP/REST. If need be, you can create an HTML client for your app as well. Know that cross-platform tools as sweet as they may sound, fail by no narrow margin when compared to the completeness, performance and agility of native tools. Xamarin, CSHTML5 and other such options serve only the simplest of scenarios, and that too as long as they do.