This seem like a good idea but in practise i am not sure how this will work for large codebase where more time is spent on understanding and guessing where to make a modification or adding a feature than writing code.

And how this will not be flooded by devs from poorest countries of the world like upwork and a lot of freelancing websites making those platforms pretty much useless for westerners.

I believe that if you can define your task in as much details, and break it to the most micro level, then it works. If on the other hand, you're issue is something like "there's a bug when sometimes an invoice is inserted with a negative amount", so yeah, this is something more comprehensive, and maybe related to god knows what in the codebase....