Parts Per Million is a small publisher with a speciality.

Solo roleplaying.

From their DTRPG page:-

Parts Per Million is a specialist creator of Solo Roleplaying tools. I believe that moving between your chosen Oracle and your favorite game should be seamless, the entire experience should feel like one game.

The solo supplements you will find here are intended to be an entry point into solo play. Every soloist builds their own set of preferred tools. These supplements will get you started, they are intended to be an easy introduction to solo play.

It is in that vein that PPM recently released an all-new resource, Basic Solo Roleplaying, or BSRP.

This rules supplement is aimed at any and all BRP Basic Roleplaying games, including Call of Cthulhu and Runequest, and anything else that is independently produced using the SRD.

The book contains a d100 oracle, a selection of muses or spark tables, advice on improv gaming and solo playing adventure/mystery games.

Solo rules are a great way to turn any rulebook into a quickstart for the GM. Rather than just making characters to get a feel for the system, you can make characters and play scenarios, all before presenting a table to your table.

BSRP and Mythras

Solo roleplaying supplements set up what are called oracles to guide you through times of no GM to make the decisions for you. You can run the game as a GM without players, more or less automating the PCs' actions, or you can run the game as a PC and let the supplement's oracles decide on the answers to their questions.

BSRP has been adapted for Chaosium's Basic Roleplaying, which means that the basic oracular mechanisms tooled for BRP ought to work for Mythras, not to mention all of the titles which use the basic Mythras and Mythras Imperative engines, such as Luther Arkwright, Worlds United, Thennla, Destined and FIoracitta.

Parts Per Million's BSRP on DTRPG

I also recommend the system-neutral PPM titles Easier Solo Play, Easier Story Craft, and Easier Encounters to add a little extra variety to your solo Mythras game.