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You could try using an ESP-32 based NodeMCU board instead of the ESP8266 based one. It has a lot more outputs.
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Instead of having to issue a command to turn it on then a command to turn it off, issue the "on" command, then wait a fixed amount of time using delay() or millis(), then issue the "off" command. I do it when I want momentary relay outputs.
Nice project and good instructable. I think you might need to add another pushbutton to the parts list. Three used on the PiFET but only two listed.