What browser are you using? Because sometimes the problem isn't even the platform or your internet, but extensions or browser settings that are freezing the video.
I've seen a lot of people with buffering even with good fiber, so you're not alone. But besides what you've already tried, test disabling hardware acceleration in your browser (go to Settings > Privacy and security > disable the hardware acceleration option). Sounds weird, but in some cases it speeds things up more than it lags. Also try opening Twitch in incognito or private mode to rule out problematic extensions - if it works fine there, it's one of them sabotaging it.
Another thing nobody mentions: test changing your DNS server (use Cloudflare's 1.1.1.1 instead of your ISP's default). Sometimes the route your traffic is taking to reach Twitch's servers is congested, and switching DNS fixes it. If none of that works, then it might be a more serious issue with Twitch or your specific route to them, so you'll just have to wait or contact support.