If you're not getting better adhesion than what you report for Tamiya acylics, something is interfering with the bond to the plastic, or bond is not being established in the first place.
The main difference in performance between acrylics and enamels is that a plastic surface to be painted with acrylics must be CLEAN—as in nearly surgically clean. Any oil, of any kind, whether from skin, mold release agent, or other source, will prevent bond. Clean the surface thoroughly, and finish with a Windex or 90 percent isopropyl alcohol wipe. Thereafter, do not touch the surface with your fingers.
The solvents in enamel paint readily dissolve most oils and mold release agents. The solvents in acrylics do not.
The other possibility is that you are applying the acrylics too dry.
I use Tamiya flat grays as primers on plastic surfaces. I have used Tamiya acrylics to camoflage my compound hunting bow. Their adhesion in both cases is superior. The only things that will remove it are Windex, 90 percent isopropyl alcohol, and Simple Green.
BTW: A strongly bonded coating over a weakly bonded coating generally results in no coating.