Copy the stop-other command to the .ddev/commands/host/ directory or the global ~/.ddev/commands/host directory (to make it work for all projects).
Now you can run ddev stop-other to stop all running projects except the one you are calling the command from.
This requires that the jq command be installed. brew install jq or see https://stedolan.github.io/jq/download/
Original idea by sceo (on Discord) and code by @rfay