One option might be to separate your loading bar from the actual scenes, so that your can render it over the canvas while Playcanvas does it’s scene change in the background.
@Kulodo133 has a really good example here that I quite like for smooth scene transitions:
https://playcanvas.com/project/779957/overview/asset-helper
Since the loading screens are all image and css elements that live outside of the application, you don’t have to worry about them existing while changing scenes. If you don’t need all of the tools in that asset helper, it is possible to separate the pieces into a more manageable script if you need.