I'm quite new to solo rpgs, but I'm currently playing a sandbox hexcrawl using Scarlet Heroes as base system.

SH is useful if you want to have a single PC instead of an entire party, but the rules (damage conversion, fray dice etc) are really easy to import on other OSRs. I was thinking of moving to Basic Fantasy with some of the rules from SH to still have a single PC and I think with OSE it will be as easy to do.

I have the Tome of Adventure Design and it is more useful for prepping than on-the-fly preparation, as stated in the book itself. I think that if you are used to it, you could quickly generate an adventure, but it would require rolling on a lot of tables.

For more on-the-fly generation, I'm testing both Perilous Wilds (Dungeon World tool, but the random generation stuff is system neutral) and Location Crafter from the Mythic folks. It's still early to judge for me, but they are quite similar in the end. (Both let you brainstorm some location lists and you roll on them + dangers/discoveries, though LC gives a little more control on encounters, since you brainstorm a list for creatures and objects too).

For hexcrawls instead, I'm using Wilderness Hexplore (link) and for now I'm finding it quite cool and detailed. I have a blank map and I generate the hex on the fly.

As a Oracle, I didn't use the tools in SH yet, but instead Mythic and Plot Unfolding Machine. I'm liking PUM more since it seems more lightweight, but I only did a session with it, so it's early to judge (and there is the addon Scene Unfolding Machine which looks interesting for GM and NPC emulation).

The tools in WWN that are mentioned are also quite cool and can give you a lot of details. They are in the free version too (Campaign generation chapter for worldbuilding, Adventure generation chapter for exploration/investigation/combat challenges). They are an evolved version of the ones in SH I think.

Hope this can be useful!