I know of your problem as well.
The issue with enforce a max limit with the auto approve is that you don't always want the first 25 signing ppl to be the actually 25 to go.
example - you have 9 ranged spots, and the first 5 ppl signing are the same class, meaning you won't get the right class mix.
So you will have to make some ajustments yourself anyway, unless you are very very lucky or don't care that much about ur class mix.
What we do is we use the autosignup system where our entire raid grp is signed up when I schedule the events.
I then allow casuals/socials to sign as well, but when they sign themselves they are put as waiting.
The day before the actual raid I look over my grp and if we still have more then 25 signed, then I waitlist the ppl I dont see fitting for the encounter.
Either way you will have to do some work yourself as I see it.
What could be really cool though, would be to have a third option when setting your signup by......
The third option should actually be a mix of the two we have today, so we both can set number of class in each role.
Example:
priest healers needed 2-3
Resto Druid needed 1-2
Warrior melee needed 0-1
Warrior tank needed 1-2
and so on and so on
If this was an option then you could work with a limit in total - but still only in a way, since it will still risk a wrong setup. But it would be closer though