Standard advice is to always book immediately and then check when onboard to see if the price has stayed low enough to justify rebooking.
However, since you are already past the opening day feeding frenzy, I'd probably just wait the few weeks, book when onboard, and then transfer to your TA. (or if your upcoming cruise was booked through DU, you should be able to assign them as the TA while onboard.)
You just need to weigh the possibility of a specific cabin you want being sold in the next few weeks and/or the possibility of the cruise selling out versus the irritation of having to put a 20% deposit down now and still put a 10% deposit down for the same cruise while you do your onboard booking. (Yes, you'll get the 20% back, but it could be an issue to some people.)
