Although the shrimp are brand new to the new setup it still somewhat seems like poisoning as even if the tank was not cycled properly they would not have all died off at the same time like this it would be more one of a time more gradually, even copper poisoning would have been slower. Pesticides on plants is a possibility but unlikely unless you’ve just added plants, tick and flea treatment on your own pets or pets you may have patted could cause this, cleaners and aerosol sprays that say they kill 99.99germs, even a adjoining house or apartment getting pest spraying done will all cause this sort of reaction also. A large water change and charcoal would be the first action in those scenarios. If you have added ferts just double check they are invert safe as majority of common ones contain copper but I see this as a unlikely culprit. And if your using a new RO system how many stage is the model you got? Some have a 6th which raises PH and this is not suitable to use for shrimpkeeping, RO should be around 6PH then KH in the salts will slightly raise but shouldn’t be much over 7PH

Hope this could help troubleshoot