I like the whole idea a lot when the mini dungeon is in the overworld, it just works a whole lot better. I will say, though, that the entire side quest is kind of... huge? I mean, you're interacting with Nigel the whole game, it seems like you're proposing. His shop is being built until you beat all the dungeons, and then you can go on a search for a mini dungeon, which you complete to find a side quest item, and in the end, after all this, you get!... a wallet upgrade you can't even take with you?
To me, it seems like maybe the quest could be a little smaller, or a little bigger, and could fit both ways.
Smaller, a smaller quest to match the reward.
Bigger, and involve Nigel in the plot better, or relegate this mini dungeon to a more relevant character. The game definitely has the theme of oppression and inequality: the poor oppressed by the rich, the gerudo oppressed by their false king and by hylian politics, and considered worse than hylians, the river zora seem to be revolting because they aren't the same as sea zora, etc.
Nigel is kind of just a guy that likes treasure.
So maybe somebody more involved, like one of Link's gang members could have a map, or something? Or maybe Nigel could be more relevant to the game, he could be slightly younger and a master thief who once lived under Faron's guidance, but grew up and moved into the treasure robbery business. Or maybe he funds the poor people, and besides running into him adventuring, you run into him pulling a robin hood. Maybe he has a tragic past of growing up in the poor side and dragged himself into the riches?
Most of those things don't really fithis goofy bumbling personality all that well, though. Anyway, just brain storming.