I did similar on Rizk, they had reel races with cash prizes(iirc they might have had 1x wager or were wager free) and not much competition each race(plus you could spin your 500 spins then start another 500 to attempt to improve your score). I would have a few hundred onsite, play only reel races daily for good freespins or cash prizes and win cash, continue to use that cash for more reel races winning more prizes etc.
I think I won and withdrew a few grand before they banned me from the reel races, didn't break any terms and they refused to provide me a ban reason(even via GDPR request about what info they held on me). I'm still welcome to play there today, just not on the races.
Bonus abuse is more as people mentioned above, say you do a 300% deposit bonus and do a bunch of $3 bets on Wild Swarm etc contributing towards the Swarm Bonus then bust that, deposit with raw cash and cheaply trigger a super bonus. That will get you banned/unpaid when you try to withdraw.