After all my problems, I’ve decided to bite the bullet and stay with my original provider. While they cannot guarantee my upload connection, at least it is a connection. With the new provider, I was dependent on the goodwill of my neighbours to even get a connection, so I’ve given it up as a bad idea.
Hopefully my provider will resolve the problems with my upload speed. In the meantime, I need some sort of working internet connection, so I’ll live with it.