The Midi action in the Cubase plugin was created to simplify the Stream Deck configuration. When using the Generic Remote device in Cubase, you must first define the commands you want to use in the Generic Remote setup. Once that is done, the plugin can read that configuration and make it available on Stream Deck without any further configuration.

With the new MIDI Remote in Cubase, the order of events is the reverse - you have to have an existing device (i.e., a configured Stream Deck) before configuring the MIDI Remote in Cubase. Maybe future versions of Cubase will change this, but it is currently not possible to create a button in the MIDI Remote configuration in any other way than to learn from an external device, and once learned, it is not possible to edit the values learned (e.g., change the CC value).

Due to this, I don't find it meaningful to adapt the Cubase plugin to the new MIDI Remote in Cubase.

Steinberg will remove the Generic Remote device from Cubase sometime in the future. They have announced that they will not rush this and will not do it until they are confident that the MIDI Remote can handle everything that the Generic Remote does today. When the Generic Remote device in Cubase is removed, the midi action in the Cubase plugin will no longer work.

MIDI Remote template

I have created a MIDI Remote Controller Surface for Cubase 12 and an accompanying Stream Deck profile. You can use these as templates for how to control the new MIDI Remote with the Stream Deck Midi plugin.

Please note that, contrary to what seems to be logical, you need to mark the MIDI Remote button (in Cubase) as "Toggle: Off" regardless of whether it should toggle or not. Set all MIDI Remote buttons to Toggle:Off, and configure the Stream Deck button to the required behavior.

You can download the template files from here.