It is ok to decline a recruiter. You can't take all opportunities, and are going to have to decide which one to take.
There will always be opportunities, as long as you keep working on yourself.
Thank them for their time.
Leave it open, that this opportunity doesn't interest you right now, and that you might reach out to them in the future.
At the moment (March 2019), the growth is really happening in Indonesia
Easier to find rocketship companies there
If you want a better salary, and better opportunities, go to Singapore