Sorry, but tight capacity is just one of the problems that many airlines face. The risk of buying too many aircraft for your slots is just that -- a risk.
Near as I can tell, the number of slots that sami's got in the system looks appropriate for that airport. Based on what little I could find in the 2003-2005 range, Don Muang had a capacity of about 60 arr/dep per hour. That works out to 30 in AWS as only departures are counted here. Sami's got it set for 24 right now, and with capacity growth that occurs every year, you'll be at least at the 30 within the next 12-15 years.
And it'd be a little difficult to move you to Suvarnabhumi as that airport doesn't open for almost 15 more years in-game.
I'm surprised that, this far into the game, there's still 7 competitors there. That's where all your slots are at. You're gonna have to work on trying to run some of those out of business and free up some slots.