Idea: use a power bank that allows in/out at the same time. It should charge both at high speed while also acting as a firewall.

This is also assuming that your powerbank can’t be hacked. In which case, god save us all.

I suspect that someone using a public usb-a charging port isn’t expecting fast charging, but rather counting themselves lucky that it works at all.

Same with Android, the various fast charging protocols use the data pins to negotiate the voltage setting and how much current the phone is allowed to draw. I suppose in theory you could make an active dongle which MITMs the data pins and strips anything it doesn't recognise as a valid fast charging command, but I don't know if such a product exists.

Except you are stuck with slower charging speeds. Usually in the places you most want fast charging but have the least trust (random public charging spots)

Most users, most of the time, will trade speed for security.