I would rather use Spotify than Apple Music, the predictions and curated playlists are far better on Spotify. But the ability to request music using voice commands as I drive is a safety issue, and I can do that with Apple. So I switched about a year ago and took my whole family with me. You aren’t the only one.
Yeah it's silly. They support Amazon Alexa, Google Assistant and Cortana. But not Siri Shortcuts!? Sort of Absurd for them to say "not right now". And even though Spotify and Apple have had a bad relationship legally, what about all their customers that they alienate when it comes to features such as this.
My only work around has been to use the URI command system for "Spotify voice" and that's if you even have it.