I don’t like the native browser interface very much. For quick work I quite like Cantor, which has Sagemath as a plugin. If you have a more complex project structure or need more graphics support (interactive 3D doesn’t work well in Cantor), Cocalc has a Docker image with good defaults, and that notebook interface is way more usable (as well as offering Jupyter, iPython, TeX and a few others natively).
FWIW both SageMath and CoCalc are part of Sage project. I’ve been using them in the past few weeks mostly because of the seamless collaboration. Haven’t heard of KDE Cantor though.