Does the first generation iPad Pro 12.9 inch have full trackpad support?

No, Only iPad Pro 12.9-inch (3rd generation or later) and iPad Pro 11-inch (1st generation or later) are supported.

Yes,

As long as your iPad is running iPadOS 13.4.1 or later, your iPad has full track pad and mouse support.

This input device support is optimized for Apple's MagicMouse 2 and Magic Trackpad 2.

All iPad models that support iPadOS 13.x can support the Apple Magic Trackpad 2. Some recent models also have accessory Folio Keyboards with Trackpad - and the 2018/2020 models of iPad Pro have the new Magic Keyboard that also incorporates a Trackpad.