I think @Dray is right that bonding is the best (only?) way in your case, once you are bumping up against the technical limits of your distance to the exchange extra lines is the only option I know.
A less good (but cheaper) way that some people (like me) achieve higher speeds is to load balance two separate phone lines (but without paying for bonding) or one phone line plus a mobile connection. It's still two costs, mind you and has some limitations compared to bonding.
In your case I don't think this would work, as if you are streaming then you need total upstream bandwidth and would need streaming software that could utilise / aggregate two IP addresses and connections - as far as I know the "usual" software like OBS doesn't do this - happy to be corrected on this if I am wrong.
Some software which uses aggregated connections works well like this e.g. Bittorrent
This kinda thing :
[youtube]https://www.youtube.com/watch?v=wGqcGeTQ7lI&t=470s[/youtube]
Anyway, I shall end this unhelpful post, sorry !
Chunkers