I spent several years working to increase my strumming proficiency. I had previously used my fingers or I used basic folk/roots strumming. I took lessons, and spent a considerable amount of time breaking down the strums and learning them. For me, I generally needed to work slowly to get up to speed, and I would use a metronome to keep me where I needed to be. My instructor would work them out on paper and also by playing.
My impression, is that for some types of music, you can set some rules for how the strumming should work, but for other types, you really have to forget those rules or they will simply get in your way. Gibson guitars has some good instructional videos on the web you might try, and Youtube has a lot, but you have to determine the quality for yourself.
Reggae strumming can be a challenge at first, but once you get it down it is a lot of fun. Some of the other strums you already know can be modified as you play to either add a strum or leave it out - and that can give your music some variety. At some point, you need to throw caution to the wind and just play it the way you hear it, and later figure it out.
A lot of it depends on the type of music you're listening to. Each genre has its own style. I think playing any rhythm is a lost art that often gets overlooked. I listen to a lot of the progressive rock from the 70's. I've heard some people do fantastic things with simple strums and others that...well, there were times when I wanted to lean over and say "...why don't you put down your guitar for a while" There's a right and wrong way to approach it.
For me, the master of acoustic rhythm guitar is Ian Anderson of Jethro Tull. I take A LOT of cues from his playing:
Often a very simple straightforward strum can carry a whole arrangement:
I agree with the notion that a strum pattern can easily carry the whole arrangement. A great example in my opinion is the syncopated rhythm in "Tequila Sunrise" by The Eagles. For me, this is a really great song that is extremely hard to strum and sing effectively and I'm a pretty good natural strummer.