What is the opinion of Reddit about the
Metal Daybed Twin Bed Frame with Adjustable Pop Up Trundle, Extendable Bed Daybed for Bedroom Living Room, Black?

IKEA isn’t the only company that sells a stacking mattress set-up like this. You can do a search on Amazon or Wayfair for “pop up trundle bed” to get lots of options. This one for example works for mattresses up to 8″ thick – which depending on your height/weight sleeping position is more likely an appropriate thickness for an adult mattress.