Hey Seth,
You are correct that TL advancement is almost always dynamically affected by performance (read time, topics entered, days visited, replies etc) but there are a couple of caveats.
Firstly, TL4 is a level that cannot be achieved dynamically – a user must be manually promoted to that level by an admin because it affords people extra power as per Trust Level Permissions Table (inc Moderator Roles).
Secondly, an admin can lock a user’s TL so that they don’t move up or down the levels. This is done on an individual basis.
You can also set the minimum trust level for a new user when you invite them.
I think the best way to achieve what you are trying to is to create custom groups outside of TLs but @JammyDodger is our resident expert.