I’ve just done this with 2 routines and a virtual contact device “flasher”
When the flasher virtual device is opened a routine auto closes it after 30 seconds.
While the flasher virtual device is open the same routine cycles turning the light on and off every 2 seconds.
I’ve just tested this having it triggered by my front door motion sensor and it seems to work .
Would this fit the bill?
Ps this solution assumes the light is already off before it starts flashing. If you wanted it to flash when the light is already on add another routine to switch the light off if flasher virtual device is on, that would start the first routine flashing.