I no longer consider Mythic GME to be a universal supplement for playing RPGs solitaire. I think it works best as it’s own, roleplay lite, solitaire narrative writing game. I think that without other people, the closest you could get to a solo RPG experience are roleplay-lite board games.

Scarlet Heroes is very much it’s own, OSR-adjacent game. You can’t play Scarlet Heroes with B/X, you have to play it on its own. It has its own systems for managing combat that are more favorable to the PC. SH’s approach to solitaire is to give the player a selection of tables to look up die rolls on.

Advanced Heroquest and OG Warhammer Quest do much the same thing. You could play AHQ just by rolling dice and looking up tables when appropriate. In fact I think that the tables in these board games are more fun as solo games than Scarlet Heroes is.