I'm big on solo play for boardgames and own many that are either soley solitaire, have a solo mode via a BOT, or are solo friendly for playing both sides - play 1 a week at a minimum. The biggest barrier for me for solo playing TTRPGs, is that I'm really only interested in soloing those I homebrew with and not those that are specifically wrtitten for solo play. I own solo companions for a number of my homebrewing TTRPGs, but those have been disappointing as they don't really provide the right tools, or enough of the right tools - if that makes sense. That's meant I've soloed few, but did complete the Vampire of Havena solo adventure for The Dark Eye and enjoyed it. For me, using a VTT seems optimal for solo play and I bought the TDE CRB for Foundry mostly for Vampire of Havena, which did play well.
I also bought the Captain's Log solo RPG for STA, as I own the STA DLC for Fantasy Grounds. Many of The Captain's log's tables look to be every bit as useful for a campaign with a party of players, so I plan to slowly import them in to Fantasy Grounds. Once imported, I do plan on attempting a solo play campaign. I havent' bought any of the Mythic PDFs, but it's something I've pondered. The Mythic GM Emulator is available for Fantasy Grounds, but it's a lot pricier than the PDF and I'm just not sure how useful it'll be, so haven't pushed the [buy] button yet.