Mildew doesn't grow in 15min, it takes hours to even smell weird and days to grow fungi. It's literally been cleaned with soap and hot water, there's not much to grow left.

I just start a timer on my phone?

I have one for both my dryer and my washer.

I load the washer then start the timer, when it goes off I just load it in the dryer then start the other timer.

It's super easy and it's no effort at all.

I had an old wind up kitchen timer with writing on it for a while but I missed it a few too many times so I switched to the phone timers.

You don’t need it to be on the appliance itself. Hook a power metering smart outlet and you can tell. Or a shake sensor. Or an open door sensor. Combine them all for a reliable effective way to tell if it’s done and the door has been opened, all for a much lower price than what those “smart” appliances charge the premium for. This way you can also choose vendors that work on local wifi/zigbee/z wave and don’t own a paper weight or part functional item the moment the vendor decides to shutdown the servers.