For a CG custom client I use a dedicated lower-resolution channel for preview with an iVGA/NDI consumer.
The templates are prepared to scale and accept a “preview” field that actually shows the “completed” status that ignores all intro animations and updates texts instantly instead of replace animations like it should on the program output.
That preview stream is then embedded in the client and by default is shown that way when you edit a template.
When you add that template to the program channel it works with all the transitions in full resolution.
I use NDI instead of a standard screen consumers for server-client flexibility. You could launch the client from any PC in the local network and preview it without any extra latency. It also lets you show alpha channel with a checkerboard background easily. Oh and generate and save thumbnails too.
This is an example of that in actual use:
sorry for the vertical video, I actually hate them (it’s the only way I found to get both monitors)