or should I just uninstall it?
You cannot uninstall Photos, it is installed a part of the System, and other applications may rely on its presence. Photos is mainly a wrapper using system libraries, and the actual application bundle is very small, just a few MB, so you would not gain much by uninstalling. But you can simply remove the icon from the Dock and ignore Photos. It is not compulsory to use it.
but when I try to import, it says I need about 300 GB of local storage. This doesn't seem right.
Photos will create thumbnails, faces, and previews. Depending on the size of the originals the size of the support files may vary between 10% and 25% of the size of the originals.
Are the photos you want to import in files and folders, or in a photo library? If they are not a photo library, why not start by importing at first only a part of your 1,5 TB, for example the newer photos, to see what will happen? Are you trying to create a Photo Library on your System drive or on the external drive? You could first import into a Photos library on your external drive and then drag and drop it to your internal, once it has been created. Then you will know exactly, how much storage will be needed.
