Have look at the stochastic section of the WRFV user guide:
https://www2.mmm.ucar.edu/wrf/users/docs/user_guide_v4/v4.0/users_guide_chap5.html#stochastic
There are three stochastic parameterization schemes available: SKEBS, SPPT and SPP.
For maximal spread, switch them all on (note that the stochastic parameter perturbation scheme SPP perturbs only certain physic's schemes
-- as documented --, so it won't do anything if you chose other physics schemes).
&stoch
sppt = 1,1,1
skebs = 1,1,1
spp =1,1,1
It is correct, that you you will get different random number streams for all schemes if you change nens for each ensemble member.
The iseed* is if you want to keep some random number streams the same, but change others (which is not what you want for maximal spread).
Please note that while stochastic parameter schemes are an effective and easy way to create an ensemble from a single initialization, they are designed to represent model error and not initial condition error, so might underestimate the forecast uncertainty.