Technically, when you update it via js you're overwriting the existing cookie with a new one. And, from my understanding, it's then subject to the same restrictions as any other cookie set client side.
So in order to have a long-lived cookie, you essentially need to treat them as read-only client side, and push any and all update/write logic to the server such that it'll return a set-cookie header with any changes you require.
So in order to have a long-lived cookie, you essentially need to treat them as read-only client side, and push any and all update/write logic to the server such that it'll return a set-cookie header with any changes you require.