Hey, thanks for your input!

I'll probably go with C# and WPF, even though Python and Qt seems very tempting right now.. getting the dust off of knowledge and read/watched a lot of stuff about MVC, MVP annd MVVM. Not totally getting it yet, but I'll continue to watch some C# basics, and then something about data bindings, which probably helps me unterstand the MVVM stuff :)

There doesn't need to be anything with Excel left, I want them to get rid of the Sheets completely! And have like a little calendar & contact manager in the app

Yeah, I know much of it could probably be done with little effort on Google Drive, but I wanna practice coding a bit and I will also integrate features to manage the contacts of all their clients n stuff :) It will have a little bit more management stuff, some stuff that does the math automatically and checks if stuff was entered correctly.. it'll be a bit more cleaner.

But thanks for the input!

Yeah, that's kind of my current approach. I wanted preferably the same language for tool and app, but I think that's only possible with Java and that's not really something I'd like to do, since I don't like Java that much. So for now, I split the languages used to create the tool, and will probably go ahead with C#. Doing a GUI with python can be a pain and I don't know if that's all, but that pretty much exhausts all option already.

Once that's done and I'm still motivated to work on that, I will see what solution/language/framework I will use to create an app. Either with Java or JS with a framework.

If you have any other recommendations or ideas, let me know! :)

Well, it's not really something I wanna use, I have to code it at work already :P I'd rather do something with a proper application and a database instead of VBA. Not necessarily for usage purposes, but also for me to learn stuff :) Thanks for the suggestion though!