Something I don't see anyone having hit on yet: Beryl doesn't like to share.
Dropping Precia and Alicia off in the DK (And it is Dark Kingdom, forget that Negaverse nonsense) might sound like a way to have a villian team up, but given Precia and Alicia's weakened state, they're more likely to be killed or brainwashed by Beryl. Even if she hears them out, there's no guarantee she won't simply use them as cannon fodder. There's no way Beryl would ever share power, that's just not in her nature. And the only reason she obeys Metallia is that she is the source of her power. In short, an outright alliance or even mutual agreement is unrealistic.
For those who want to debate on who is more powerful, let's face it, Beryl may not be as impressive as some other villians, but right now, Precia is barely alive.