Good way to improve cornering stability is to add some stiffener bars. Especially like Myvi the hole of the rear hatch is huge, this should be the first place you stiffen. After that can look at the front struts, arb etc. Next can look into progressive springs if you want to maintain some comfort but get less roll during cornering. They're cheap to get at kereta potong and if you smile they'll let you try a few until you find one you like.

Increasing rim size will increase FC more than getting wider tires. Less comfortable too. Wider tires will increase traction more than a bigger rim size. Bigger tires will get you better traction during cornering but it will not necessarily improve how stable it feels. Myvi suffers from understeer and from my experience the rear upper bar helps the most. Also it is a very tall car, so for stability maybe you want to lower it or at least get a lower seat.

Switch off the foglights will add 25hp.