Well the sacking consistently yields about two hundred to three hundred florins on most of the smaller towns, so not only is it not worth it, but you can't afford to piss off the pope by killing catholics. It takes a while to get a diplomat to Rome to start the ol' gifting fest, so I wouldn't want to risk excom. while you're in your "fragile" state. Getting excommed also leads to lower loyalty in your settlements, something this little rush hinges on. (your alliances early on not only provide money, but guarantee a little ceasefire while you snatch up rebels left and right).

Definitely don't assault Bruge, as the sallying forth means that you can pick off the pikes with some merc crossbows.

I see your point on killing off mercs, but your force wont depend on them, they will be supplimented by them, so you aren't taking a huge economic hit too early (no frankish knights kind of units, just crossbows and spearmen early on). Since most of the towns you take wont have military facilities, you wont be able to replace the lost troops from assaulting right away. After the towns start falling left and right, money wont be your issue but troops definitely will.