Cloudflare doesn't officially support SSE, but if you send keepalives events every 15 or 20sec or so you can reliably use SSE for 40 min + in my experiance.
No server traffic for 100+ sec officially results in a 524, so you could possibly make that keepalive interval longer, but I haven't tested it.
Make sure to have the new style cache rule with Bypass cache selected and absolutely make sure you are using HTTP/2 all the way to the origin.
The 6 connections per browser limit of HTTP/1.1 SSE was painful, and I am pretty sure auto negotiation breaks, often in unexpected ways with a HTTP/1.1 origin.
On top of the comments below about SSE, I'd also point out that Cloudflare is doing some interesting stuff around serverless resumable websockets. They also have stuff for WebRTC.
