COPY (login issue): I would say that the system needed for more group detail even D&D, is usually too clumsy for solo aka CYOA kinda works. There is a reason, why Fighting Fantasy & Lone Wolf series held it simple, and that reason is NOT that the authors were dumb or sloppy.
I had to cut-down the D&D 5 rules, getting rid of time-wasters and superfluous extras, much like two decades ago with the WoD system spammed by desperate officials trying to renew their only successes.
Here is legally free stuff you can download, some meddling with the official, predefined Fighting Fantasy system (Like The mysterious case of bodies in the docks by Simone Osborne).
Rolemaster had a thief adventure for one player plus GM. I never played it, but considered it a possible shortcut to 'initiate' a newbie player into a decently useful rogue (trap disarmer, ambush spotter, and loot-box opener). 😉
Chaosium SHOULD have some free downloads for the Call of Cthulhu system, as even their cost-free section is pretty filled.
RISUS - The Anything RPG offers some cool solo-adventure ( I remember a halfling aka hobbit thief vs alchemists of Evil sect), and group potential.