The two main metrics in the choice of who can use more powerful subliminals seems to be sanity and experience.
Sanity is the more challenging to assess, but as a good starter, experience is easier to judge based on the date of the first subliminal purchase.
Ofc not fullproof since who knows how much one has used a sub but still it’s the most accurate metric I can think of.
Edit: Also a system of gradually unlocking access to different subliminals depending on purchases.
Example: someone who only has experience with LotS or BDLM even after a long time doesn’t get access to higher tier subs like Emperor Galaxy or Khan Multiverse unless they also have experienced mid tier subs.