It's been about a year and a half since Google Assistant became available on iPhone, but it's always been way more work to use than Siri because iOS users can't make Google Assistant their default voice assistant. That's not changing, but Google Assistant is now a bit easier to use on iOS.
The latest update to Google Assistant for iOS adds Siri Shortcut support. The app will even prompt you to set up a Siri Shortcut when you launch after updating. Just record a phrase, like "Hey Google", and then when you call up Siri, you can use that phrase to launch into the Google Assistant which will be waiting for your voice command.
Do keep in mind that if you want to do the whole process using just your voice, your iPhone will need to be unlocked to allow Siri to launch Google Assistant. So again, using Google Assistant on an iPhone isn't going to be as smooth as it is on an Android phone, but this is definitely an improvement. It's also kind of strange to say "Hey Siri, Hey Google," but hey, it is an option for folks who prefer Google's voice assistant over Apple's.
To give this feature a try for yourself, grab the latest version of the Google Assistant app from the App Store.