big tires, bigger rims, add UR strut bars

u can change to gas type/adjustable type (but it maybe too stiff for u), change wider tires maybe 195/55 or 195/50 or (205/55 if can fit) with maybe 16inch rims if u want to where original for avanza is 185/65/r15...where the 65 series is more to comfort, so when u drive at highspeed it tend to float abit/bounce..... and try to add UR strurt bars on the front,back....sure ur ride become more stable and can corner with much confidence compare to stock avanza...but of course cant be compare with sedan cars..la

depends on the personal preference.....for me bigger rims and wider tires sure gives better stability...i had saw quite number of avanza where they change to wider tires and with bigger rims take corners like sedan car... ....
pricing wise..im not sure but sure cost few thousands if u want to change all those....

maybe u can start with absorber 1st den only wider/better tires tires....rims not necessary...

tall,lightbody weight,small tires,and suspension setup.... .....even 120km++ i feel its gonna fly off anytime soon...