More light. Check the roots if they are winding around the pot it is time to repot.
Mint will choke itself. When it looks nice and full it is time to repot.
I think you have too much shade the mint is very lanky. In morning sun it will be fuller and shorter. In full sun it will do well once it gets used to it but in very hot places the edges will burn. Mint likes a rich moist soil. Use fertilizer, it doesn't care what kind.
you can cut the sprigs and plant 4-5 in a 4 inch pot. It should be rooted in about 2-3 weeks.
Happy gardening in Hawaii. Gardens are where people grow.