I'm not sure why the casino is complaining about fluctuations in bet size. What use is having a variable bet size on games if you're not allowed to use it? Win or lose, the casino is only too happy to take your money. I fail to see how any player gains any true benefit from changing bet size. That RTP is still going to get you in the end. Regardless of the size of your bets. The player does not gain any extra RTP by changing stake. So how are they gaining an advantage over the casino?
Let's say you deposit 150 and take a 100% match of 150%, total 300 balance and 4500 to wager at 30xbonus.... you start with 6.25 and increase to 12.50 and make it quick to a balance of 4500 with still 3000 left to wager.... now you set a bet size of 1$, chose a low variance slot and put it on autospin to complete the wagering....mathematically you end up with a nice balance to withdraw
This is what casinos call 2-tier betting, whereas i call it smart betting when you don't want to risk too much of your balance whilst still wagering.