Designing QuickCharge device
slow charging the battery pack with its ~3W and also cramming as much
  energy as possible as fast as possible in case when wall power is
  available


Some/many powerbanks will do just that. I.e., they take as much power as they can up to their designed maximum. Well-behaved devices take care not to overload the USB port, others (cheaper ones) don't care. And because they don't expect to get all the current they could handle anyway, most will work at basically arbitrarily low power/current if more is not available.

The system self-regulates/-stabilizes when the power bank tries to draw more current than the USB port/charger will provide, in which case the voltage from the port will drop, which reduces the current the power bank draws for charging and thus stabilizes at the minimum of the power the power bank wants/needs and the power/current the port can supply.

You may well find that a not-well-behaved powerbank violates the USB standard by charging from a USB port at, e.g., 700mA and 4.3V. I.e., it draws more current than the standard permits (500mA), which results in a voltage drop to below USB standard voltage and hopefully won't damage the USB port.