If you power your Pi via USB just use a phone USB bank that works via passthrough. (I don't actually have one to recommend but just one that can charge and discharge at the same time).
> one that can charge and discharge at the same time
I did some research into this as well and I'd gladly consider this as an option but most power banks can't do this. And the ones that do don't tend to advertise it. Every time someone says they found one that can do this, in some forum or whatever, they are no longer available to buy.
