I may be completely stupid but these instructions by Rumble either completely unhelpful or something on my end isn't loading content in that site. But if that's the actual instructions to get OBS to stream to Rumble, I get why just upstreaming it would've been user-friendly.
But apparently I'm a fascist for this type of wrongthink.
What's confusing about using RTMP or a virtual camera? It's extremely straightforward. I genuinely do not understand how you could possibly need more instructions than this. Do you want screenshots or something?
Regardless, this just means that Rumble needs better documentation. It's clearly possible and fairly easy.