This is weird to me. In my current game, I have about 1400 realm size, control everything from Mauretania through Rajastan, and have granted pretty much every county I can get my hands on to dynasts (as well as the duchy and viceroyal sultanate titles). I rarely have problems with decadence. I got up to 30% in one unpleasant regency, where I had a couple decadent family members I couldn't imprison, but I managed it by pursuing a couple holy wars. I usually run somewhere around -20 from imprisoning family members, but that's easily made up by my huge opinion boosts from piety, prestige, Hajj, Sayyid, etc, and I intentionally run at -40 tyrrany so as to not 'waste' the opinion surplus.
A couple important things: you get one free imprisonment on a dynast who refuses to 'straighten up', regardless of wether you demand it or one of their family members. If you /release/ a decadent prisoner, you can't reimprison them, though. This makes Ramadan a trap - that 'release prisoners' event can destroy you pretty quick. For shedding holy decadence, holy wars are by far the best mechanism. Heathen neighbors are a wonderful thing. If your realm is large enough for decadence to be a concern, you should have plenty available.
You /do/ have to pay attention, though. When I turn the audio off, and miss the notifications for failed straighten up, my decadence hits the teens pretty quick. I think the biggest thing decadence needs, gameplay-wise (and really, /I/ don't care much about historical veracity, so long as its fun), is improvements to notification and tracking. 'Decadent family member' should be a popup on the top of the screen, like righteous imprisonment.