I do dance the core rumbas to a degree, but typically only in the context of Casino/Timba, although I do plan to organize social rumbas at some point

You have correctly noticed the sparse nature of rumba "patterns" - in yambu and guaguanco, there are basically a few basic steps (floreo in place, basic sidestep, the traveling cachan step, and variations of these, such as the marco-abre marco-cierra, or marco-abre cachan sidesteps). Then, there are a few of typical stylings (hand patterns, turns) and some vacuna/botao patterns in guaguanco, and that's pretty much it! But the core of the dance is just pure improvisation, innovation, contact and emotion based on this minimal set of building blocks. I typically compare yambu and guaguanco to slow/fast blues

(and columbia to jazz or progressive metal

)
Rumba Columbia may be a bit richer in "known" patterns, but on the other hand, it doesn't look that way as it encourages, no, REQUIRES constant innovation and individuality as a solo competitive dance.
I believe it is also important to realize that rumbas are really street, people's dances - there is no RIGHT way to style it, everybody will do a significantly different variation, but preserve the basic feeling of the dance. Do not get stuck at observing famous current festival teachers and think their style is the one "authentic" thing - they have often stylized it waaaay too much. Rumba has a specific african aesthetic that does not always have to be "beautiful" in western eyes, which some teachers are unfortunately trying to achieve, frustrating students and taking the fun out of it.
Best,
Jan / Cubana Ljubljana