When you unsub, you will become a Preferred player with a few restrictions. For example, you can only play 8 of your characters (but you can choose which ones to play), you have a maximum credit limit of 350k (everything above is kept until you resubscribe), you can no longer do operations without buying a pass, you have a maximum limit of weekly warzones, to name some of the restrictions. But you will get all of that back when you resubscribe - there isn't really a disadvantage in letting your subscription lapse.
You keep access to all content released until the end of your subscription, including the KotFE chapters. If you only want to play through the story, a single month of sub time will get you plenty of story to play through. Any future content will require a subscription again when that content comes out, or any time afterwards (this may change for the upcoming KotET expansion; we don't know details yet).
My recommendation would be to definitely get a subscription, even if just for a month. It only costs like $15 but you get access to plenty content to play through at your leisure, even after your subscription, and as a Preferred Player many F2P restrictions disappear.
