Female guppies killing males

Hey, this is my first time keeping female guppies. I've always kept males, but was inspired to get some females and try breeding them.

I have a 30L tank, plenty of hides and plants, water parameters are perfect, temp in the green. I had 4 male and 8 female guppies in the tank (1:2 ratio like suggested), along with a cherry shrimp clean up crew, and Garry- an albino corydoras who was previously tank mates with my male Betta. (Betta is fine, removed his buddy because he became a father).

I assumed guppy breeding would be much easier compared to Betta breeding, I looked up several written and video tutorials, and even consulted the manager of my favorite aquarium store. Everything all good by their standards.

It's been a month now. I don't think any of the females are pregnant, and all 4 males are dead. After the first 2 deaths I removed Garry, just in case he was the cause- stressing them out or something.

Garry was not the problem.

I stayed and watched the next time I fed the tank, and watched as the females ganged up on the remaining 2 males, nipping at their longer fins and chasing them away from the food. I started dividing the tank for feedings, assuming it was just aggression over food, the females were bigger and faster and I thought the males might not be getting enough food. After feedings I would remove my makeshift divider.

Everything seemed to be going great, for 2 weeks there was no death. And then disaster struck. I was out all day at doctors appointments, doing groceries, baby book time at the library with my son; a very full day that meant I was away from the house for a solid 8 hours. When I returned the 2 males were both dead, torn up and floating at the top of the tank.

I am devastated.

Since then the female guppies have even started targeting my cherry shrimp, and their population in that tank has begun to dwindle.

I've never even seen this kind of aggression from my Bettas.

I honestly don't know what went wrong or what I'm supposed to do now. I just have 8 plain looking female guppies taking up one of my best tanks, and I'm too afraid to put any other fish in there with them.

What do I do?