@popey I'd tune in for a bit if it was Owncast, but likely not Twitch or YouTube.
Someone suggested multistreaming from OBS elsewhere in the thread; if you haven't seen it already, @muesli created prism a few years ago and it worked quite well last I tried it. https://github.com/muesli/prism
It sits on a box on a datacenter, you stream to it, and it forwards that stream to YouTube, Facebook, Owncast, PeerTube, whatever simultaneously. Prism wouldn't solve the split chat problem, but it would the bandwidth problem.