Most models of iPad require a Power Adapter rated at 12W (i.e., 2.4A @5V) or greater to reliably charge. 5W Power Adapters intended for older models of iPhone (and early models of iPad) are inadequate to charge modern iPads; if the iPad charges at all, charging will be very slow.
Recent models of iPad and iPhone support USB-PD (Power Delivery) - and can operate/charge from a variety of input voltages/current. The stock Apple 20W Power Adapter can supply 3.0A @5V and 2.0A @9V (2.22A @9V when operating from an +200VAC-supply).
By contrast, Apple’s 30W USB-PD Power Adapter can output 1.5A @20V, 2.0A @15V, 3.0A @9V, 3.0A @5V. You should note that when connected to a device that does not support USB-PD, the Power Adapter will default to 5V operation.
Be aware that Apple devices only support standard USB Power (5V) or USB-PD. Other proprietary charging standards are not supported.
Apple has announced release of two models of USB-C 35W dual port Power Adapters. More details of their specifications can be found here:
https://apple.news/Ae_fO5lM9Rwe3b-in53Z3Ag
Other third-party manufacturers market compatible multi-port Power Adapters for Apple devices. Perhaps look at Anker and Belkin - but ensure that the model that you choose supports USB-PD.