How to automate settings upon opening an image in ACR?

Erik’s correct and that is what I do too. With an image you haven’t edited yet, make the adjustments you would typically apply to every image, then save that as a new Develop preset. If you always change the raw profile to a specific one (such as Camera Neutral or Adaptive Color), make sure your preferred profile is part of that preset too.

But there are additional levels to this that can save you even more time.

Having a preset is fine for manually selecting a bunch of images and applying it. But you can even skip that step if you make that preset your default that is applied to every image you edit for the first time in Camera Raw. That means you don’t have to do anything except open the image, and it will apply your default preset and profile. For details on how to do that, read this Adobe article:

How to Create Custom Raw Defaults in Adobe Camera – Julianne Kost

If you also use the mobile Lightroom apps, you can import that favorite preset into the desktop version of cloud Lightroom (not Lightroom Classic), and that will sync that preset to all mobile clients. You do have to set it as the default preset in the Lightroom app on each device. I have set this up, so now…

…my favorite preset and profile are applied by default to every unedited raw image I open in Camera Raw or import into Lightroom Classic, no matter how I took that photo: Using any of my digital cameras, and (if I used the Lightroom camera) using my phone and tablet too. If I take a photo with the Lightroom app camera, my default preset is applied as soon as I exit the camera and view the image I took, right there on the phone.

So the only manual edits I need to make in Camera Raw and Lightroom are the edits that are individual to each photo.

Note that you can have different defaults per camera and per ISO setting. For example, if you wish the images you shot at ISO 12800 on your Canon had Denoise applied by default, you can do that. Per camera defaults are explained in the link above, ISO adaptive presets are explained at this link:

ISO Adaptive Presets – Adobe help article

…Or should I be batch applying these kind of settings in Lightroom instead? 

By @unknownsailor

The only reason to batch apply in Lightroom is if that’s the software you’re using at the moment. Because everything described above works in Camera Raw too. (Except for the part about syncing the preset to mobile clients, you do need cloud Lightroom for that.)

Create a preset using these settings (and any other settings you might want applied by default). Then go to default settings and select your saved preset as your default. 

Then your settings will be applied automatically whenever you open an image in ACR. You can set defaults globally or for specific cameras as required.