There is a simpler way. Use your device (iPhone, iPad,etc.) to play your Spotify from your Spotify account and then select the echo device you want to play on under “connect to device”.
Make sure that your device and echo are on the same WiFi network.
We have one amazon account, multiple echo devices, tied to one Spotify account (account 1) and If I want play my Spotify account 2 then I just open up my Spotify account 2 on my iPad and select the echo device to connect to. The echo device acts like a blue tooth speaker and even “obeys” my voice commands like “next” “stop”. Works for me!
Hope this helps. Cheers.