iPad Pro batteries are very high capacity - and requires a power source of sufficient power output and capacity. Many PowerBanks intended for iPhone will have capacity that is far too low to offer practical benefit.
Your sixth generation iPad Pro has the biggest internal batteries of them all. Your iPad will optimally charge when connected to a USB PD (Power Delivery) power source capable of at least 20W or greater output.
For a PowerBank, you’ll be looking something that supports USB-C PD output, possibly with multiple outputs, at least one of which should be capable of at least 20W sustained output. Depending upon your needs, a useful portable PowerBanks will have a capacity of ~26,000mAh - this equating to the maximum battery power capacity that can be legally carried on a commercial airline.
Always choose a mainstream branded manufacturer - mophie being a good example.