There are a couple of places in Singapore apart from Goethe Institute that provides quality German classes.
1. Learn German https://www.learngerman.com.sg/
This is a website that is a platform that will help you get in touch with a German language school. You simply submit an inquiry and they will get in touch.
2. Inlingua https://www.inlingua.edu.sg/
They are a language school that has been quite long in the game and they teach a variety of languages. They have the experience to teach well.
Another language school that has teaches a variety of languages. They are located in Thompson and East Coast. Maybe difficult to get to without your own transport.
4. Agape School of Education https://agape.school/
This is the language school that I myself had the chance to experience. I took up German classes with them 2 years back aiming to pick up the language and apply to a Uni in Germany. They managed to get me to be Goethe Zertifikat C2 certified within that short period. It was really tough but we pulled it off.