A Bluetooth mouse, Bluetooth trackpad, or Magic Keyboard (with built-in trackpad) can be used to interact with Affinity Photo using an on-screen pointer. It can be used in addition to your iPad's touchscreen and (optional) Apple Pencil. Your iPad must be running iPadOS 13.4 or later.
Apple provides guides about how to connect a Bluetooth mouse or trackpad and use it with your iPad.
If you have difficulty finding the pointer against your artwork, or it gets in the way, the latter guide includes instructions for customizing its size, color and auto-hide duration, as well as its tracking speed and scrolling direction.