External GPU? Order of magnitude more.
Not really. Getting a decent enclosure costs money up front, but the enclosure should last across multiple video card upgrades. Plus if you get the right enclosure it can serve as a laptop charger and a dock—both items are kinda pricy.
There are a lot of options better than the BlackMagic that Apple sells. The only thing the BlackMagic has going for it is that it can power a Thunderbolt Display, but not being able to upgrade the card inside makes it a much, much more expensive and short-lived option.
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Target Display Mode hasn’t worked on the iMac since the Late 2013 model. It has never been supported by the 5K iMac or the iMac Pro.
That is/was such a weird feature. Did anyone make heavy use of it?
I still use it, but my use case is weird. My dad's 2010 27" iMac has some hardware issues—the speakers don't work and it's no longer getting software updates from Apple. I turned it into a Time Machine server, moved him to a laptop, and he still uses the display. The 2010 model is still a pretty darn nice 1440p display.