I like QtCreator code completition for QML too. I would have the same in VSCode (where I edit python files), but until now I haven't found a valid plugin.
I use often QML Live for live update of my components/pages (dual monitor is a plus).
My main reason against the Qt Quick Designer is the possibility to build responsive (or multi-size display) UI..... but may be I don't know all its features well.
Here some consideration about it: https://extenly.com/2021/04/22/qt-quick-designer-in-practice-facts-and-myths/