I would like Low Power Mode to be the default state of my device.
When I get to 80% battery it toggles itself off automatically.
How can I stop it turning itself off?
How can I always keep my phone in low power mode (or some other equivalent)?
There is no way to achieve this functionality as it will always turn off at 80% as the purpose of it is to extend your phone when at low battery.
Low Power Mode reduces or turns off Email fetch, Hey Siri, Background app refresh, Automatic downloads, Wi-Fi associations, and some visual effects. If you can live without having those features available on the phone, you can keep it on low power mode. But after you charge your iPhone to 80% or higher, Low Power Mode automatically turns off, so you'll have to turn it back on again.