8 decks take patience
I play KO and of necessity play mostly 8 decks, so I understand what you are saying. The game can be beaten, but not for much money unless back-counting is a reasonable option and you have the bankroll and table limits to get big bets out.
In answer to your question, do not raise your bet at -18. That's only 10 points above the IRC, and even if you were to get there after only one deck you are still at a disadvantage. Wait for the Key Count,or use entry points of -14 after 1 deck, -11 after 2 decks, -8 after 3 decks, and the KC from there on.
It takes patience and it can be boring.