@schizanon Not sure why you think that @Vivaldi's "PWA support is non-existent", but it does have it.
To install a PWA through Vivaldi, you have to right-click on the tab (while that tab is active). There, you may get two options: "Install <PWA name>..." and "Create Shortcut..."
You use the former for websites that actually support running as PWAs, and the latter for every other website.