The Apple USB-C Digital AV Multiport Adapter can be externally powered - using the incorporated USB-C pass-through port.
There are many third-party USB Hubs that have a USB-C connection for iPad plus a USB-C PD (Power Delivery) pass-through power connector to which you can connect your USB-C Power Adapter - along with the hub itself incorporating a combination of additional USB-C ports, USB 2.0/3.1 Type-A ports, Ethernet LAN, HDMI, VGA, SD and microSD card-readers.
Perhaps consider Kingston, Anker, Hyperdrive and Satechi - although there are many others from which to choose. A Google Search for USB-C Hubs compatible with iPad will find a number of articles and recommendations.
Speaking of personal experience, the Kingston Nucleum has proven itself to be particularly flexible for connection of high power USB storage devices - featuring twin USB3.1 Type-A, downstream USB-C and SD/microSD card slots, HDMI, plus a power pass-through USB-C port that can accept +45W from a suitable USB power source.
If looking for an HDMI Adapter, be aware that very few USB-C HDMI adapters/hubs support HDCP 2.0 - absence of which will inhibit the ability to output copy-protected HD media content from iPad to a connected monitor/TV over an HDMI connection.
The Apple USB-C Digital AV Multiport Adapter does support HDCP 2.0, but perhaps lacks some of the additional the flexibility afforded by some third-party USB-C hubs:
https://store.apple.com/uk/xc/product/MUF82ZM/A
Note also that some iPad models that feature USB-C also support DisplayPort Alternate Mode over USB-C. This protocol allows direct connection, with an appropriate Thunderbolt cable, to a monitor that supports DisplayPort - removing the need for HDMI.