Unfortunately using an NFC tag does not solve the problem. That is because the Shortcut app that is triggered by tapping an NFC tag simply activates an existing Home scene or automation (that does not let me set a timer). It does not create a Homekit event that I can use to trigger a timed automation.
So my only option seems to be to have a second outlet that I turn on using the NFC tag and that, in turn, triggers an automation that turns on the humidifier outlet for a set time - as in the previous screenshot. That second outlet would likely not be used for any other control (at least Ikea smart outlets are relatively cheap!).
Also, not surprisingly, the Apple Watch SE does not detect NFC tags - I have to tap the tag with my iPhone.
Snapshot from Shortcuts app (see 9to5 article for details)
