The thing you need to look at is the DAME handbook:
https://www.casa.gov.au/sites/g/file...i_f13_5348.pdf
CASA is a world unto itself. It holds opinions that are at odds with both contemporary medical practice and overseas aviation medical departments (notably the FAA).
This quote from the DAME Handbook pretty much sums up CASA's stance on everything, not just medications:
As a general principle, DAMEs should assume all medications are hazardous for aviation operations until demonstrated otherwise
They pretty much think that anything medical is cause for them to withdraw your medical until you prove otherwise.
Don't assume that there is any logic in their policies. Never for a moment believe that they are your friend.
Grommets are very common in children with chronic ear infections. I assume they are put in under general unaesthetic which means you need to admit to it at your next medical or lie.
I'd take the advice of your (non DAME) GP and the specialist performing the procedure. You may require a letter from your specialist saying that it will not affect your ability to fly. Go to CASA only when you have all your ducks in a row.
My guess is that CASA will refer it to one of their subcontracted medical assessors. I have had an experience where this was an overseas trained doctor giving an opinion that outside their specialty that was in conflict with the opinion of a notable specialist in that field. So, my advice is to start with enough information to put you on the high ground.