The diamonds from steven universe were a great looming threat but were terrible as characters. Also, Steven Universe as a whole was crazily overrated
You're not alone in this!
I merely regard it (and by extension Future) as a good show that had good moments but ultimately tried to play up its own mysteries too much by waiting to say the crucial story beats pretty late (even if some of the foreshadowing was pretty mind-blowing, looking back). The Diamonds felt woefully underutilized, and when they FINALLY got shown more they felt like little more than props via heel-face-turn to make Steven look better rather than actual characters. Best show ever? No not even close, I still like the DCAU shows, Phineas and Ferb, and Wander Over Yonder more, even if they are (mostly) simpler.
I binged most of the show until that long Season 4 hiatus, and even then it felt kinda slow (let's wait a whole season for Jasper to get out of the sea) and messages that, while I commend the showmakers for being bold with, felt overbearing / overshadowing actual plot at times even if some hit the mark fine (Steven's characterization in Future).
While it might be an odd comparison, I'd argue you can present messages in a much more subtle way and still make great stories - A LOT of the old Spongebob episodes I never realized had aesops until I explicitly read about them:
So I won't get too deep into it, I'll sum it up: Steven Universe (Future) is a good show with good voice acting, music, art, etc - but it also shows how sometimes a complex scope can swallow a show whole, for better or worse. I think the movie was really good at catching people up to speed - it's not easy to summarize all that continuity and still have a relatively coherent plot!