Hello thelittlestwho
There are a few ways you could do this.
If you say "Go to sleep" voice control will stop listening until you say "Wake up". This does not turn voice control off but it will not listen for commands until you say the "wake up" command. Appreciate this does not turn off voice control but has the same effect.
OR
You can say "Turn off Voice Control" and "Click confirm" and voice control will be turned off.
To turn it back on you have to say "Hey Siri, Turn on voice control"
OR
If you want to start voice control using an on screen icon - create a Shortcut in the Shortcuts app and add it to your Home Screen - you can then toggle voice control on and off by pressing the icon, or have voice control turn on when you open a specific app. The shortcut is a one line entry see screenshot attached
OR
You could turn on the Accessibility short cut (Settings>Accesibility>>Accessibility Shortcut and then select voice control). If you then triple click the side button it will then either toggle voice control on or off, or present a menu depending on how you set it up.
OR
You can turn on Assistive Touch (Settings>Accessibility>Assistive Touch) and customise the menu to include voice control. You can then access the voice control option through the assistive touch button
Hope that all makes sense and one of these options work for you.