For the products you've purchased that came with an A-to-C cable, have you tried using them with a plain C-to-C cable? In my experience usually that will "just work". (Maybe there are some power banks out there that don't support outputting power through their USB-C port. If those exist I've managed to avoid them so far.)
At least on Android, there's an option to change the power direction (IIRC, it's in the same place where you choose the data connection mode). I have to use that option to switch the direction whenever I use the USB-C cable to charge my phone from my laptop, otherwise it tries to charge the laptop from the phone.
That's not just a convention, it's a vital part of the specification :)
USB-C devices go to significant lengths to make it hard for users to violate that invariant using an unsafe combination of otherwise safe adapters – that's why there is no USB-C-to-A adapter, for example (because that would let you build an A-to-A cable, and USB-A hosts are not required to detect possible loops/short circuits; USB-C hosts are).
I believe an USB-C device port is easier to implement than a host/power supply port, since the host is only allowed to supply Vbus after ensuring that it's connected to a device (by actively probing the resistor connected to the CC pins etc.), whereas a device port only needs to present that resistor. After all, even a passive C-to-micro-B cable can do that.
No displayport or smartwatch here, and my power banks all came with A-to-micro or A-to-C cables (mostly the former) - there's a convention in that ecosystem that power flows from the A connector to the other end.