What is the easiest way to transfer photos from iPhone?

Another way is to connect the iPhone to your Mac with its USB charging cable, open Image Capture, select all photos and videos and save to a Folder on the Mac. Then you can do what you want with them. You can also use Image Capture to delete selected photos from your iPhone.

The above is based on the assumption that you do not have iCloud Photos activated and optimize storage on the iPhone enabled.

Use an external drive, and plug it into the phone.

Select the pictures; choose Share; scroll; choose "Save to Files."

[if you have "Optimize Storage" turned on, then you may have to wait for the pictures to load from iCloud]

When you get the Files menu, you can choose the external drive.

I would do them in smaller groups-- not try to do them all at once. 3600 pictures will be less than 100GB, probably.