With the help of Tomas and Jan, we've got this sorted out.
pipewire 0.3.15 did help with Chrome, Firefox, and OBS being now able to
share the screen. It doesn't help with UX of pipewire portal dialogs but
this is something I can live with for time being.
I noticed a pipewire update today and that greatly reduced the list of
dependent packages getting in the way of a dnf swap:
The paprefs package depends on pulseaudio-module-gsettings and is not
a problem, more like a collateral casualty.
The non-rpmfusion pulseaudio-module-* packages seem to be part of the
problem with an explicit pulseaudio dependency. They all come from the
pulseaudio SRPM, but it's not clear whether the scope of the
pipewire-pulseaudio package includes those modules. Playing a bit with
dnf repoquery it doesn't seem to be the case, and the other
pulseaudio-module-* packages I didn't install on my system seem to
share the same behavior.
Same explicit dependency with the xfce4-pulseaudio-plugin package.
I'm now much closer to giving pipewire a try.