Hello - First off the disclaimer still stands. I don't know if Pioneer and Onkyo have ever released the system images for this player, so doing this is totally at your own risk.
But Xposed Framework allows you to enable and disable mods to the Android system very easily without changing things yourself manually. It is slightly outdated nowadays since Magisk took over the android rooting scene. But there are still apps like Amplify which use Xposed.
What Amplify does is keeps track of everything that is keeping Android awake and draining your battery. Wakelocks and Alarms will periodically wake your device or use more energy than you'd think. Things like periodic checks to the internet or just randomly checking for updates can kill your battery. Many of these things have been resolved in the latest Android devices but since this DAP uses an older Android 5.1 system, many of those are not implemented. So, with Amplify, you can see what is keeping your device awake. It'll also let you know if something is risky to turn off or not.
But when you think you want to disable or limit something, you can set a time limit on it so it can't check multiple times within a certain time period. This helps you determine if it is important or not, so you can always disable or make the time limit longer later. Eventually, you can disable enough non-critical wakelocks, alarms, etc that your phone will not be constantly active and draining battery.
There are a few unknown wakelocks I had disabled that I ended up having to re-enable because they caused issues with Play Store log-in, but I've been successful limiting or disabling many of these wakelocks and now my battery lasts 15-17 hours using balanced music - I can easily last a full day at work.