it could be interesting for Librem5?
I might bet PureOS need to stay within firmware-linux-free package borders, however perhaps someone have more time (as I don’t) to test (manage UEFI firmware updates) following options:
One (current Debian main repo):
$ sudo apt install fwupd-amd64-signed
$ sudo fwupdmgr --help
$ sudo fwupdmgr get-devices
sudo apt install gnome-software-plugin-snap
sudo snap install fwupd --classic
Three (future, as requires Debian Sid main repo):
$ sudo apt install gnome-firmware (GTK front end for fwupd)
$ flatpak install org.freedesktop.fwupd
or (as OP initially linked to):
$ flatpak install org.gnome.Firmware
If not pleased:
$ flatpak remove org.gnome.Firmware
$ flatpak remove org.gnome.Platform (3.34)
$ flatpak remove org.freedesktop.fwupd
This is only a draft (not an answer) from my side, but will be pleased to get some news if this (which option, if any) can be useful, as OP asked/recommended (even if fwupd needs to be avoided). Also, if this should work (with time) just with UEFI someone might give an answer anyway.
P.S. @antpanlinux, maybe you should move this to PureOS category (topics).