How to configure a Bluetooth keyboard to open and close a specific app?
Does your keyboard have media buttons by any chance? If so you can use something like Tasker or Automate (Free) to programatically set actions. I happen to have Automate on my phone and just checked you can use media buttons as a trigger e.g. pause, play, next, previous, eject etc. Then you can hook this up to start apps and voice searches easily - the interface is pretty straightforward if you haven't used one of these apps before.

If your keyboard doesn't have media buttons a generic bluetooth remote of some sort could work (try ebay/aliexpress etc.)

Quick google search says Tasker can't capture generic keypresses but that might've changed since 2015.