Cutting the cake 30 mins before dessert is served - will guests leave super early?

No one will leave early if the music is pumpin and open bar flowing. We are having our cake cutting right after dinner which is at about 645. But I can tell you no one is leaving early because they are coming for the dancing and for all of the traditional festivities.

I think having the DJ announce dessert is fine!

We did this and no one left. It's common in our social circle that the cake is dessert so I've never seen people leave after the cake cutting.

What you are describing is from the era when a cake and coffee reception was the norm. In modern society where cake and coffee only is absolutely unheard of in most circles, guests stay as long as they are enjoying themselves. However if you are not serving the cake to your guests, don’t announce it because then you will have guests upset that there isn’t enough to go around. Because some people actually do look forward to cake and don’t get excited over alternative desserts. Are guests being served dessert to the dinner tables? In that case, nothing needs to be announced.