Bank first, then SD is good for both max play time and battery health, as the charging of the SD battery results in both energy loss as heat and minute degradation of the battery.

Many wrong answers here. Most devices have what’s called a power path. When a device is powered via a cable and the battery is full, the device battery is bypassed and the electronics powered via the charging port directly.

There are always losses in both charging and discharging a battery. Discharging the power bank first and the SD battery second avoids the charging loss from the SD battery.

You should avoid keeping either battery at sub 20% state-of-charge for long periods, if possible.