Well, the x30 wagering requirement in slots is unpleasant, but it's worth noting that this is far from the highest requirement compared to other gambling platforms. If I am not mistaken, there are gaming platforms on this forum that have the x40 wagering requirement for the welcome bonus.
30x-40x wager requirement is standard number in this gambling industry, it is because the chance to complete this wagering requirement is so low.
Even under 30x (20+) will still need a big of luck from player's side in order to be able to complete the wagering requirement because slot game can be very wild game which may give players really bad playing session even after thousands of spins.
The idea of this kind of promotion is something common, maybe bethog need to have different offer for their active players only with lower wagering requirement but keep the 30x wager requirement for new players.