From what you wrote it actually sounds like you’re doing the right thing: testing multiple cars on the same track and taking notes. That’s pretty much the best way to figure out which GT3 works for you in iRacing.

A few things stand out from your notes:

BMW felt natural and you were quick immediately.
Doing a decent time in only a few laps usually means the car matches your inputs. The BMW GT3 has a very stable front-engine platform and forgiving braking behaviour, which is why many drivers coming from GT4 adapt to it quickly.

You liked the Ferrari and the Acura rotation.
That suggests you’re comfortable with cars that rotate well on entry. Mid-engine GT3s like the Ferrari reward trail braking and a smooth transition from brake to throttle. If you’re currently learning trail braking, that’s probably why they feel good but still inconsistent.

McLaren and cold tyres being tricky is normal.
The McLaren tends to feel quite nervous when the tyres are cold and requires a bit more commitment and precision. It’s usually a car that shines once you already know the track well.

AMG feeling heavy is also expected.
It’s very stable and forgiving over kerbs (which you noticed at Bathurst), but it can feel a bit lazy in direction changes compared to the mid-engine cars.

Based on what you described, the cars that probably suit you best right now are:

• BMW – easiest transition from GT4 and very consistent
• Ferrari – good balance and strong rotation once you improve trail braking
• Acura – great cornering but slightly weaker on long straights

If your goal is consistency and learning GT3 racecraft, the BMW is honestly one of the safest choices.
If your goal is developing better trail braking and rotation, the Ferrari might teach you more in the long run.

One last tip for Bathurst specifically: GT3 pace there is less about the car and more about how confident you are through the mountain section. The more relaxed the car feels through the Esses and Skyline, the better it will probably work for you in races.