What platform/technology/framework to use. I am lost. :-(

NickJ911  Uno Platform  may be a good fit for you; it is open source, production ready today (MAUI is not) and proven with real apps. It leverages .NET 6, C# 10 and Xamarin (not Forms but the stuff in .NET), uses one of the best designed .NET UI frameworks: WinUI 3, which you use directly to build for Windows and then UNO has ports of WinUI 3 for Android, iOS and more on top of Xamarin.

UNO also lets you target the browser via webassembly with the same UI framework and source code (so not a mix of 2 frameworks like Blazor and MAUI)

On top of that, if you like C# and would prefer declarative C# to XAML for your UI, you can create your UI (and so your entire app) in C# 10 with C# Markup 2 for WinUI and Uno Platform (disclosure: I am the author of that library - let me know if you need help using C# Markup)

I have about 20 years C# .NET experience, last 8 years in Xamarin, and specialize in the developer experience of .NET UI frameworks.