Yes, that is the consensus of why Qt sucks - it's a massive framework that tries to do everything at the same time with a massive toolset of in-house libraries. This is inherently tied to the revenue model of the Qt Company - sell custom modules that work well with the Qt ecosystem at a high enterprise-level price. I also wish to just use the "good" parts of Qt but I can't, since it already has a massive QtCore as its dependency.
However, there is still no cross-platform framework except for Qt that can actually do the most important things that a desktop framework actually needs: an actual widget editor, styling and theming, internationalization, interop with native graphics APIs (though I have gripes with their RHI system), etc. That's why I'm rooting for PanGUI (https://www.pangui.io/) to succeed - it pretty much completes all the checkboxes you have, but it's still WIP and in closed alpha.
> I hoped someday Flutter might be mature enough for desktop development >> Anything that forces a specific language and/or runtime is dead in the water.
Yeah, but at that time I thought this was at least better than wrangling with Qt / QML. You can write the core application logic ("engine" code) in C++ and bind it with Dart. There are already some companies I've seen gone a similar route with C# / WPF.