Avanza is a bubble car, too light for its own good. Haha.
Changing suspension and everything will not improve much, because the car will wobble with it anyway. Drive with more loads or people in it... Add stabilizer bars or the good old strut bars to reduce rolling too.

Good luck yo.

Buddy,
Suspension does the trick. There are options posted here from cheap to expensive.

Cheapest: Get a mechanic to replace your stock shock fluid with thicker suspension fluid.
Next: Change to stiffer aftermarket shocks front and rear.
Next: Full set suspension spring and shocks front and rear.
Next: Add stabilizer bar and tower struts for cornering stability...

Hope this helps.