Oh right, should probably report back on that.
So, I tried everything y'all recommended. Out of those, I like #QKSMS the most so far; it's quick to load, the interface is easy to navigate, and overall gets the job done. #SimpleSMS would be my second pick, but adding an "add contant photo" button above every message in a conversations makes scrolling more annoying.
I'm keeping both of them for now, just in case QK turns out to be less accessible when it comes to MMS messages. Haven't tested that yet. But I think it'll serve me just fine.
Thank you everyone for your help!