Spotify connect lives in your AVR. It pulls directly from spotify server. Your tablet is only acting as a remote to queue server requests. No cable required.

No audio data is transmitted from your tablet to AVR.

No need to mess with firestick. As mentioned, the Denon is pulling directly from server.

http://manuals.denon.com/AVRX3500H/eu/en/SXQWSYnpjwflgv.php

Wifi is entirely fine. Only need 1mbs or so for spotify streams.

It isnt video, so not very “expensive” on your network.

Welcome to streaming! And excitingly soon to be CD quality!