Thank you for the great logs and very thorough reply! Here's what I'd do!

On step 3 I went to /etc/pacman.conf and I couldnt find [multilib] I could only find [multilib-rel]. So I added ‘[multilib] Include = /etc/pacman.d/mirrorlist’ like the guide said. Then I did sudo pacman -Syu and the update failed.

holo-rel is up to date core-rel is up to date extra-rel is up to date community-rel is up to date multilib-rel is up to date multilib.db failed to download error: failed retrieving file ‘multilib.db’ from steamdeck-packages.steamos.cloud : The requested URL returned error: 404 error: failed to synchronize all databases (failed to retrieve some files)

Since the [multilib-rel] section is already there, I'd just make sure that's uncommented. I wouldn't add the [multilib] section. Seems like that could be a typo perhaps. In the upstream Arch mirrors, it's [multilib], so they probably got it mixed up. If this works, I'd write the author of this guide to have them correct it.

Then after that I went through the steps and got to installing lutris. Which gave me this error for every package required. Do you want to delete it? [Y/n] y error: python-lxml: signature from "Felix Yan " is unknown trust :: File /var/cache/pacman/pkg/python-lxml-4.9.1-1-x86_64.pkg.tar.zst is corrupted (invalid or corrupted package (PGP signature)). Do you want to delete it? [Y/n] ^C

Sounds like the PGP keys are either not synced or out-of-date. Assuming the Deck works like upstream Arch, you should be able to run:

pacman-key --init
pacman-key --populate

After this, try your pacman command again.

More information here: https://wiki.archlinux.org/title/Pacman/Package_signing

Hope this works! Let me know if you run into any more friction and I'm happy to help!