Super viable. I'm aware of several companies using the LGPL license for their closed source desktop applications, including my own. Don't let the other replies scare you. No one will call and threaten you and you don't need a lawyer. For cross platform native desktop applications, Qt is far and away the best option. QtCreator is pretty awesome too.
You will be missing a few modules/plugins/tools which are commercial-only (I cannot seem to find the actual list right now though) but other than that, the LGPL was made exactly for that reason to allow linking an open-source library (Qt) into a closed-source program.
As long as you don't modify the Qt source (or you modify and re-distribute it per LGPL) you should be safe (IANAL obviously)
As long as you dynamically link and don't use the non-free components (charts, etc.), I think it would be just fine for a C++ or Python app. However, personally, I am leaning towards Flutter atm. They have decent emulated native looking controls for Windows/Mac, seems to have a strong cross platform focus, strong Rust integration via a bridge library, and has a better license with good free charting options.
I have a project I built using Qt version 5. It seems to work pretty well. I didn't upgrade to version 6 because it was initially missing some modules I was using, but it sounds like they have now ported those to their version 6 code. When I get some cycles, I will have to look into upgrading.
The LGPL license allows you to make closed applications without problems if you comply with the rules. But the Qt guys will call you and make threats (very nicely). Real case.
What? It's doesn't take a genius to understand it. They clearly label what is and what is not LGPL. It doesn't hurt to double check the source files of course and have redundant confirmation. You don't need a lawyer for everything in life.
I don't need a lawyer to use a LGPL lib. Considering how many threats you have to go through to get the LGPL version, I need one because Digia is likely to disagree with my interpretation.