Yes, the message now goes into junk mail of Gmail by default instead of being completely blocked. This means that if there are any stranded Gmail loyalty tokens that they are again accessible! Yay! And the situation is better because there is now a SPF record for mud.shatteredkingdoms.org subdomain and the message is emitted by the IP listed in that record. But the message still doesn't pass alignment: the FROM address of the confirmation email is no-reply@shatteredkingdoms.org and the returnpath/mailfrom is mud@mud.shatteredkingdoms.org. The domains after the @ symbol have to match to be aligned. Alignment is what is needed to get the confirmation emails to get into the Inbox by default in Gmail instead of Junk Mail. You could change either one to match the other at this point.

I'm not sure where the FROM is generated or if it is easy to change in your particular case (traditionally it is) but here is a description about how to change the returnpath/mailfrom in Postfix: https://logic.edchen.org/how-to-rewrite-return-path-and-sender-address-in-postfix/ - you'd want a rewrite mapping of mud@mud.shatteredkingdoms.org to map to no-reply@shatteredkingdoms.org instead.