If you are receiving a Quality score of 0, it is absolutely your connection to the servers causing the problem.
The MOST COMMON ISSUE (and reason I restate it here) is streaming over wifi. If you are, stop and run a cable. You said you've tried everything, but it is so commonly THE cause, that I cannot assume you aren't.
Past that, install a utility called PingPlotter (there is a free version). Point it at the ingest server you use while streaming, and It will monitor your connection hops for packet loss and latency spread. The last node (the Twitch ingest server) will always be 100% packet loss, as Twitch have disabled ICMP replies. This is expected.
Leave Pingplotter running while you stream. You're looking for the first node where the PL column is non-zero, or for a node with a very large latency spread bar. It isn't a guarantee, but points at the most likely problem node causing your throughput issues.
If it's in the first hop or two, chances are good that the problem is inside your house; a bad network cable or switch, a faulty modem/router.
First two or three, it may be the connection from your house to the local concentrator; this would be up to your ISP to fix.
Past that, it could be in your ISP's intranet, anywhere up to the local backbone uplink. If you take a screenshot of the PP window after streaming for an hour or so (with problems!) and paste it here, I can look it over and advise further.
This is all circumstantial suggestion, no hard 'this is it' evidence, but can point VERY strongly toward where the problem lies, and give you ammunition for the next time you speak with your ISP.