this should be simple but I can’t get mathematica to simplify a radical or a set of radicals and express them in the traditional way

for example by simplifying this

$$text{FullSimplify}left(left(3 a sqrt(4){18 a^4}right)^3,a>0right)// TraditionalForm$$

results in this

$$81 2^{3/4} sqrt{3} a^6$$

or this

$$27 18^{3/4} a^6$$

but not this,

$$81 sqrt(4){72} a^6$$

which is the final and most compact

There is some way I can simplify even more and from the results in a traditional way, this is just a small example.