I've tried using this mod with the Steam Deck and I have a few issues:

1. Virtual cursor - why can't I select buttons like with the Midnight Controls mod?
2. Rear buttons - these are not detected
3. Text input - opening the virtual keyboard causes the text box to become deselected with no way to reselect

It would be great to have these improvements as I would like to play FTB modpacks on my Steam Deck and this mod is easy to get added in.

Are you using Steam virtual gamepad?

No, I have it set to "gamepad with joystick trackpad" so it'll work with the Midnight Controls mod in my Fabric instances.

using any steam input mode that uses "A button, B button, etc." is a virtual xbox 360 controller. so you are using steam's virtual gamepad. an option for having the extra buttons work is mapping them to unused keyboard binds (Ex: L4 is numpad1, R4 is numpad2, L5 is numpad4, R5 is numpad5) but then you need to use the keyboard controls menu and bind them.

Following up to mention this is the method to use Controllable with Steam Deck, which has been supported for a while.
https://mrcrayfish.gitbook.io/controllable-documentation/guides/how-to-connect-a-gamepad/method-4-steam-virtual-gamepad