If the data occasionally changes, you can just respond to the update request with a 409 Conflict reporting the problem and allowing the user to fix it. If it's changing so fast that it requires real-time updates, the user can't probably keep up either, so you should re-think your mechanism.
Note that SSE belongs to the content layer (HTML) rather than the transfer layer (HTTP) so its specifically about notifications to active clients about content changes to resources they're observing.
Here's someone describing their experience with it: https://medium.com/axiomzenteam/websockets-http-2-and-sse-5c...
Also, SSEs make it essentially impossible to implement features like real-time user online/offline presence and automatic renewal of authentication tokens (without closing the previous real-time/SSE connection).
What is there about what you're describing that's more difficult with PATCH than alternative protocols?
Unless of course you have two users directly editing the same field simultaneously, but this is a direct conflict case that any underlying implementations will have to have their own strategy for—HTTP wouldn't be any different to anything else here.