^ That worked for me! I wish it was a part of the official documentation though. In all my research I haven't found that repo on my own. Thanks for your reply!

For anyone with the same problem, basically, try to run
curl -1sLf 'https://dl.cloudsmith.io/public/moonlight-game-streaming/moonlight-qt/setup.deb.sh' | distro=raspbian codename=$(lsb_release -cs) sudo -E bash
instead of
curl -1sLf 'https://dl.cloudsmith.io/public/moonlight-game-streaming/moonlight-qt/setup.deb.sh' | distro=raspbian sudo -E bash

from moonlight-qt.

The following are my own instructions for OSMC + Moonlight-QT + Moonlight Kodi plugin.

Installation

• Configure /boot/config-user.txt

• (If not fixed on the OS level) Add the osmc user into the input group

sudo usermod -a -G input $USER
sudo reboot

• Install moonlight-qt as per instructions

# The following command will try to resolve the distribution release
# If that does not work, use 'codename=buster' to override the value
curl -1sLf 'https://dl.cloudsmith.io/public/moonlight-game-streaming/moonlight-qt/setup.deb.sh' | distro=raspbian codename=$(lsb_release -cs) sudo -E bash
sudo apt install moonlight-qt

• (Optional) Install Docker
• Install addon

# Check the website for latest release version (https://github.com/veldenb/plugin.program.moonlight-qt/releases/latest)
VERSION=0.4.3; wget https://github.com/veldenb/plugin.program.moonlight-qt/releases/latest/download/plugin.program.moonlight-qt-${VERSION}.zip
# Install the downloaded addon package using Kodi GUI. For convenience, download into $HOME folder
# Remove downloaded plugin archive, as its no longer needed

• Configure addon
• Connect a keyboard to RPI (controller navigation does not work for all buttons)
• Configure moonlight-qt settings and add a host

Updating

Plugin needs to be updated manually by following the steps outlined above.
From official addon repository docs:

When you want to update Moonlight you can use the update menu in the add-on settings and press "Update Moonlight to the latest version".
The plugin will update Moonlight and will notify you when it's finished.

Monitoring

tail -f ~/.kodi/userdata/addon_data/plugin.program.moonlight-qt/moonlight-qt.log

References:

• https://github.com/veldenb/plugin.program.moonlight-qt
• https://github.com/moonlight-stream/moonlight-docs/wiki/Installing-Moonlight-Qt-on-Raspberry-Pi-4

from moonlight-qt.

I updated the installation steps to add codename=$(lsb_release -cs). Thanks for the suggestion.

from moonlight-qt.