If you specifically want to introduce yourself, there is a forum category for that: General / Introductions
so a topic in there would be appropriate.
I am firmly of the opinion that you should always know how to start from scratch on any computer. So, go right ahead and learn how to do that.
Hint: the instructions for doing that mostly assume Debian family, so if you mostly use arch on your desktop / laptop, there may be a bit more work there (or else produce a Live Boot USB flash drive for a Debian family distro, for the times that you need to reflash your Librem 5, or back it up, or fix it up, or get in remotely … all from a host computer).
passwd
?
Just to be clear, I took login screen to mean the unlock screen where it wants the password for the purism account i.e. the default account that the phone GUI runs under.
This is different from the screen where it solicits the LUKS passphrase, which always assumes full printable ASCII i.e. is a fairly standard on-screen keyboard and can access all printable ASCII characters.
The choice of whether to use LUKS or not is made when you download the disk image to flash, and is made by using an argument to the flash script (--variant plain or --variant luks). However I believe that (if you have a current version of the script) it defaults to --variant luks so you are good to go with that.