Hi Jordy,
Clearing bonuses isn't always that easy in the first place.
Some of them expire if you dont bet enough to clear the bonus in a given time.
Also playing online can be tough (I dont know if you played much online)
I dont think there is any way to build a huge bankroll quick in poker. It is pretty competitive out there.
You tax question is very specific to the Netherlands so I dont know how many people here can help you. All the best
-lemon