I want to empty the gallery on my iPhone. Will that delete them from iCloud too?

Tap Settings > [your name] > iCloud > Photos. Tap Sync this iPhone. In iOS 15 or earlier, tap to turn on iCloud Photos. Select Optimize iPhone Storage to save space on your device.

Manage your photo and video storage - Apple Support (IN)

Enabling iCloud for Photos, Notes, Videos, Reminders, Messages, and more, as depicted in the provided picture, means that if you delete any or all data from one device, it will also be removed from iCloud and all other devices linked to the same Apple ID. Likewise, the reverse is true as well.

xadgar wrote:

Is there a proper well-defined way

Yes, there is.

If you delete Photos & Videos from the iPhone the same is deleted on iCloud Photos too. So...

1. Microsoft support says --> Import photos and videos from an iPhone to PC
2. Import to your Mac --> Transfer photos and videos from your iPhone, iPad, or iPod ...
   1. Connect your iPhone, iPad, or iPod touch to your Mac with a USB cable.
   2. Open the Photos app on your computer.
   3. The Photos app shows an Import screen with all the photos and videos that are on your connected device. ... 
   4. If asked, unlock your iOS or iPadOS device using your passcode.
3. Upload to iCloud Drive (Not iCloud Photos) using the Files App.

Now, you can safely delete from the iPhone as you have the Photos/Videos on iCloud Drive and your Mac/PC

xadgar wrote:

. However, I do want to completely empty my entire gallery on my iPhone and keep everything on my 2TB iCloud. Is it safe to do that?

No, No, No. If you delete it from your iPhone it will also be deleted from your iCloud.