How can I update ONLY the security portions of iOS 16.5, without downloading the entire 2.75GB onto my iPad? Because I am running low on storage space.

That is not possible.

See especially “If there’s not enough space to update” here.

If your iPhone or iPad won't update - Apple Support

Apple has made its design decisions. This is the way iOS updates have worked since the original iPhone was introduced in 2007.

There should never be a reason to accept only a portion of an iOS update.

Regardless whether you’re updating or not, you should keep at least 6GB storage free at all times.