Have you talked through your DDs diet with the HV? It would be worth checking if she is getting enough solid food through the daytime, that her portion sizes, meals, snacks and so on are enough.
9oz strikes me as a massive bottle of milk at 12m+. Especially if there will be more milk through the night too. I think the 1-3y Toddler Milk (the stage 3 formula) tin recommends 6oz bottles.
If your DD is up and screaming anyway after a small amount of milk in the night, if it was me I'd just stop all night feeds and do it all in one go. Rather than prolonging the agony, get it all over and done with in one go.
As to what to do - that's where the gradual withdrawal comes in. If you weren't doing night feeds then no need to wait for bottle to warm, so you will get to her sooner.
I am not sure what the millipond version is, but my whole ethos in establishing independent sleep is GW based so I can help with that. Basically the first step (which is where you are) is doing anything to get baby to sleep. Preferably with baby lying in cot and you leaning into the cot, patting, shushing, reassuring firm hand on chest/back, tickling, whatever is needed really. It will take a long time and have lots of tears. But you can't move onto the second stage until you have established a 'lots of reassurance and plenty of your presence' way of getting her to sleep without milk.
Once established, then you start making gradual changes to lessen the dependence on your presence and reassurance - you gradually withdraw. But you can't start withdrawing until you have a way to get baby to sleep in the first place.
If I am perfectly honest, while I am a huge advocate of GW, I believe it is an ethos that starts from newborn baby and is very, very slow and gradual over the first 6 to 12 months. I do not believe GW can work effectively if started later in baby's life once bad habits are engrained. Or it would wake a very, very long time.
Where is a thread on the sleep board called "What Worked For Us" that has a Gradual Withdrawal method that worked for an older child. It might be worth a read of the opening post.
Your HV probably isn't allowed to recommend things like Controlled Crying or Cry it Out, so is giving you the gentle option. But it might be more effective for everyone if you consider the sum-total of the distress caused to baby and you over the many months that GW will take, compared to the awful but quick option of CIO or CC.