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.

It does add a slight extra layer in that they have to both have a compromise for whatever chip is controlling your bank and a compromise for whatever phone is attached which is more difficult to pack into a small controller chip. Although I'm willing to be a lot of power bank controllers are similar across the market which narrows that difficulty.

If your power bank is qi (MagSafe) to your phone, it will probably be fine given the lack of a wired connection.

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.