I use my iMac Pro with usb-c to hdmi cable to an Eizo to broadcast in second monitor.
Is there any better solution ?
Aja or BMD broadcast is useful or not ?
Is it better to have a external device like Aja or BMD to keep GPU perfomances ?
Nothing wrong with that. Critical color is better via Sdi but unless that’s incredibly important to you are likely fine. Once you split over sdi then you’ll need more sdi to audio de embedders and a studio monitor controller and other misc novelties.