I need to use Microsoft Fonts since I use my Deck as a laptop replacement and I don't know how to download/install MS Fonts (preferrably MS 10/11). I don't have a MS image or CD to rip it from there :/
SteamOS uses inmutable partitions, you can't install things using pacman safely. For OP's problem my first thought is using a virtual machine or a whole different OS. Maybe there are fonts in Discover/Flathub, which is what SteamOS is designed to use, but I don't know if that could work.
I've seen this but the AUR isn't open by default and there's a reason why it's not opened. I've read that it can lead to faulty behaviour and software not working properly