Here’s how things went for me on CachyOS (in case anyone’s interested):

Installed deps:

sudo pacman -Syu \
 curl git base-devel \
 meson ninja \
 gtk4 gettext dbus libverto openssl systemd-libs \
 desktop-file-utils \
 python-dbus-next \
 zip

Then built from source:

git clone https://github.com/linux-credentials/credentialsd
cd credentialsd
meson setup -Dprefix=/usr/local build-release
cd build-release
meson install

Got it running fine with the custom Flatpak Firefox build — worked across all sites I tested. On Android, I did see some cosmetic error messages when using hybrid transport with BLE, but functionality wasn’t affected.

Couldn’t get the webextension working in either standard Firefox or Flatpak Firefox (the latter expected, since it can’t hook into the portal anyway right?).

Linked the native messaging host with:

mkdir -p ~/.mozilla/native-messaging-hosts/
ln -s /usr/local/lib64/mozilla/native-messaging-hosts/xyz.iinuwa.credentialsd_helper.json ~/.mozilla/native-messaging-hosts/

Probably some dumb mistake on my side, but do not worry about that :)