How do I make a good solo quest?

I wish I understood what your frame of reference is, because it sounds like a decent question. I’m just afraid that I don’t know anywhere near enough about your game. However, in a rational game and world, I would expect entrepreneurial enterprise to be generally successful (more often than not, that is) if planned well for the parameters of the game / market and if executed with a strict policy of exchanging value for value, and as honestly as possible, given the parameters and rules of the game. Sometimes cheating, robbing and pillaging works in the short time or in desperate straits, but I wouldn’t want that to be a standard modus operandi; I’d look for a new game. But maybe that’s just me.

Remember that aside from bashing monsters, there are a variety of other possible challenges, including them needing use their brain to figure out the answer. Decode languages, persuade NPCs to do something, etc.

For example, the character needs to use that ship to get to a specific destination, so they need to figure a way how to persuade all the other crew members to alter course. They have to work up the ship hierarchy to get enough respect by physically putting key players into their place, and persuading others to accept his/her leadership. I would set the quest so that the PC needs to persuade, say, 6 key figures on the ship, and each of them are persuaded in a different way. Once the PC wins their support, I give them that player to use as an NPC.

Have you read Robin Hobb’s Liveship Traders trilogy?
There’s much to go on when it comes to hierarchy on the ships in those books. (Also great read).

p.s. haven’t DMed in a long time, this is just my two cents