I think if radyven aa buff didn't give 500% multiplicative dmg, but plus 400% stacking dmg that would already solve the problem.
When using a haste or no trainer at all, the damage would stay exactly the same at 5x the original damage, but when using a dmg trainer the pet would do 500% (for example) from trainer+ 400% from radyven as damage. That'd still be a significant dmg buff and a lot of damage, without being broken.