The startlxqtwayland script sets XDG_CURRENT_DESKTOP="LXQt:$COMPOSITOR:wlroots", in my case compositor is labwc. Does : mean something or this is just a custom thing to allow you to initialize the right backend? I think that is what broke portals loading in my case for the Wayland session as they all have UseIn, i.e. UseIn=LXQt.

I used the systemctl --user edit xdg-desktop-portal.service and I override this:

[Service]
Environment="XDG_CURRENT_DESKTOP=LXQt"

And I have this in ~/.config/xdg-desktop-portal/portals.conf:

[preferred]
default=lxqt;gtk;
org.freedesktop.impl.portal.FileChooser=lxqt;gtk;
org.freedesktop.impl.portal.Screencast=wlr
org.freedesktop.impl.portal.Screenshot=wlr

So now the situation is better, but I still have to do systemctl --user restart xdg-desktop-portal once before I have all the portals loaded, the service is initially started but without any portal implementation loaded.