Not sure if this helps, but the PyQt5 PyQtWebEngine (separate module) module includes QWebEngineView under QWebEngineWidgets just like PySide6.
See for example on how to use it via the plugin_utils here:
https://github.com/Sigil-Ebook/Readi.../plugin.py#L57
The Windows and macOS Sigil builds under Qt5 included it as part of the standard install beginning with Sigil 1.60 back in April of 2021.