You've misunderstood the meaning of 30/30, it's nothing to do with how many members you have.
The first number is how many seats you have, and second number is how many seats you could have, which is based on guild level.
Max seats is guild level +10 (capped at 30) so when you create a new guild it starts at 10/10 so you can add up to 10 members, if you level that guild up twice it'll go to 10/12, if you then buy a new seat it'll go to 11/12, then you can have max of 11 members.
Since your guild is already above level 20 and all seats have been purchased, it'll always be 30/30 unless the guild loses stability and drops below level 20.
This has 0 impact on daily GP cost.