To be clear, you want something that connects to a dry contact-type input on your pump, rather than something that’s actually providing power to your pump, correct?
If holding the button down for an unknown number of seconds doesn’t cause a problem, for a few dollars less you could use something like this, and program it (probably using WebCore) to turn off “immediately” after ST turns it on, but the amount of time for the commands to make it would be variable.
https://www.amazon.com/Remotec-Zwave-Contact-Fixture-Module/dp/B00913ATFI
Another option is a button pusher like a Switchbot but then you need their hub to integrate with ST and you’re back in the same price range as a Mimolite.
To my eyes, the Mimolite is the best option as it’s designed to do what you’re asking and you’ll probably get a more reliable result while spending less time trying to make it work than the other options. And you might find a use for the input, like monitoring the pump to confirm it’s working as commanded or tracking runtime.