Just to chip in,
Submitted via ETA app for my entire family, 1 spouse + 2 kids ( all SG) except myself who is MY. All of them got instant approval, whereas i got a notification to provide additional supporting documents. Have to go Immi online website to follow up, submit bunch of stuff like your employment status, financial doc, itinerary, flight tix, etc etc.
No need to pay anything extra when submitting supporting documents. Submitted everything on the spot, with exception of flight tix because not yet buy and travel date still 3 months ahead. Will update if i get any response.
My advice is, don't duplicate and apply your visa again especially for the subclass 600 which cost AUD 150 unless you are clearly rejected.
There is no reason to pay anything more beyond the AUD20 ETA fee because malaysia is clearly eligible for one.