If you have Sonos voice control installed on your Sonos product, you can play audio/set a timer using a voice instruction.

See this link:

https://support.sonos.com/en-us/article/list-of-voice-requests-for-sonos-voice-control

Why do you have to login to the app every time? 

To turn on the 15 minute sleep timer.

once music is playing, and the 15 minute sleep timer is used, after the 15 mins expires and the music stops, the sleep timer also disables itself (resets to continuous play for the next time play is pressed)

I’d like the device to have an option for a redundant sleep timer that can be set and maintained even after the timer expires until it is manually turned off. 

I use the sleep timer also for same reason, but i just open the app..which is instant.

I use something called  SoCo - runs on a Raspberry Pi and every night continues play from a very large playlist at a set time, it also sets a sleep timer to fade off.  Totally automated but needs a little programming knowledge and the times , length of sleep timer for me are fixed.

Have not looked at it for a few years so it’s probably horribly out of date version wide and I should update it… but it’s not broke!

There’s a 3rd-party iOS App too called Soro, which is a plugin for Apple iOS shortcuts that can do this aswell and automate it for a set time each night, but there’s a slight leaning curve for that App too. A screenshot example is attached. It’s mostly drag and drop to setup a timer - so no coding is required.