Hi! I've never really done growl stuff on UTAU, but I still want to help. I'm assuming you've seen
this tutorial already? I'm also assuming that you've already tried using flags since you've already tried a plugin (I'm an UTAU-synth user so no plugins for me hahah)
Generally I would try to get that effect during mixing instead of within the program by using distortion similar to the ones used for electric guitars. I also recommend looking at recordings of people (irl) doing the kind of growl effect you want and analyzing what the wavelength is doing (in a DAW) and then analyzing what the wavelength of the UTAU voice you're using is doing normally to see ways you can possibly replicate that sound.
I find that growling is extremely difficult for voice synth overall, even vocaloids, so don't feel discouraged!