Knowing the limits before the first spin
Before you start a session, you decide your stake and how long to keep spinning. Neither choice changes when the bonus round arrives. Its timing is not known in advance, and the result of one session is random, whatever the RTP of the version you play. Two sessions at the same bet can end at very different balances, even with the same RTP.
You decide what to trust by reading the game rules, because they state which RTP version Retrobet Casino runs for this game. No other page or lobby tile replaces that statement. Compare the figure in the rules with any number you saw elsewhere before you place a bet.
Choosing a bet sets the ceiling of your win, not the result you will reach. Big Bass Splash pays up to x5,000 the bet, which is A$25,000 at the example top bet of A$5. That ceiling is reached very rarely, so treat it as a limit on the game, not a target.