HeikoXT I thought of this before but unfortunately, iOS is very restrictive when it comes to reading NFC tags.
Some older iPhone don't support reading NFC at all or you would need to launch the app before the app is able to read the tag - this defeats the entire purpose.
Newer iPhone might be able to read NFC tags - but it requires additional user interaction "clicking on a notification" in order to have the timer actually start - also very inconvenient.
Next problem is writing the NFC tag. This feature was only introduced by Apple with iOS 14 and only is available for iPhone 7 and newer (my test device is a iPhone 6s and therefore doesn't support the feature - so I am unable to test it).
For all these reasons I currently don't plan to add such a feature