Development Framework for Desktop App with GUI

I’m tasked with developing a commercial closed-source application for desktop with GUI. I’m looking for a framework I could use.

Must have:

• Free or very cheap to use (Qt is not an option)
• Fast to execute, one execution of some external tools that need to be monitored may take days, hence fast execution is invaluable.
• Allow Protecting source code: I’ll be importing proprietary third party software (even if possibly refactored to a different language) that can only be distributed in protected format. I also don’t want customers to see implementation details.
• Allow creating GUIs
• Allow creating and updating scientific graphs.
• Allows developing for Linux

Nice to have:

• C++ as language
• Good documentation
• Widely used on GitHub/StackExchange
• Allow displaying and updating simple 3D graphics.
• Allowing to develop for multiple platforms.
• Statically linked libraries preferred, so I can have a single executable for the whole app.

Context:
I’ve been working with wxWidgets, but I don’t seem to find many questions on SO, nor plenty of code on GitHub, plus the documentation is confusion for me, and everything seems to have been made more than 5 years ago. I’ve considered using Qt, but my company does not qualify for their small-business plan, so it would end up being too expensive for starters.

The app I’m developing will interact with a lot of C code, and should run "fast" (which makes python a little undesirable), have a GUI and be preferably complied in a stand-alone executable. I cannot create a server-based application to run as a service, I need to deliver to the customer’s PC, which will be used in locations where internet access may be limited or non-existent.