So I'm using the sportsbrooker BetInAsia, but if I bet on say O/0.5 goals and the outcome is determined really early, I still need to wait until the end of the game before the bet is settled.
Yes, that's correct.BetInAsia is a broker, not a bookmaker.
Even if the outcome of your bet (e.g. Over 0.5 goals) is decided early in the match, BetInAsia, as a broker, won't settle the bet until the game officially ends. That's because they simply forward your bets to professional bookmakers like Pinnacle, and those bookmakers don’t offer early settlement or cashout all bets are officially settled only after the final whistle