The GOR tours are pretty exhausting - around 14 hours, of which approximately 10 of these are spent on the bus. Phillip Island tours are "better" in that there's less time on the bus (maybe 5 or 6 hours) and more time on the ground, but they always get back to Melbourne late because of the penguins. May is pretty good - penguins arrive around 1730 so the tours get back to Melbourne around 2030. The Grampians is 3 hours from Melbourne plus half an hour to get to Mackenzie Falls, so maybe 8 hours on the bus. To do two of these tours back-to-back would kill me.
I suggest that you do a 2 day tour to the GOR and the Grampians because they are both west of Melbourne (Phillip Island is south-east). You'll save at least 6 hours of bus travel by avoiding backtracking, and you will get to spend the night out of the city so may have the chance to hear kookaburras. The following tours I found with the help of mr. google. All spend the night in Halls Gap which means you'll have a close encounter with the hundreds of kangaroos that graze there in the evenings and early mornings.
And if you have booked non-cancellable accommodation in Melbourne, I still think it's worth forfeiting the price of accommodation and doing an overnight tour.
https://melbourneaustraliatours.com.au/public-tours/2-day-great-ocean-road-and-grampians-tour/
https://sightseeingtoursaustralia.com.au/tours/2-day-great-ocean-road-grampians-tour/
https://wildlifetours.com.au/tours/melbourne/great-ocean-road/2-day-great-ocean-road-and-grampians-tour/
https://greatoceanroadmelbournetours.com.au/tours/2-day-great-ocean-road-grampians-tour/