I think I saw a video on Snazzy Labs YouTube channel about that. One of the boards he used required way more work than I would be comfortable doing! I still have a 27” 2010 [or late 2009?] iMac with a gorgeous screen that I need to repurpose into a display. I really wish Apple still made 27” iMacs.
They do…$2,000 27” monitor!
EDIT: $1600 or $3300 display.
Yeah, but that’s not an iMac. 
I’d love a 27” iMac! The displays are just displays - no computer inside for that money.
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Similar experience with the fusion drives here too on mid-2010s era iMacs. I looked into the SSD swap out procedure and the removing the front glass part looks difficult. The lazy way out is to just get a decent USB 3 external SSD and install MacOS and your GarageSale library on that. Technically a little slower than the internal transfer speed of the Fusion Drive, but I’ve found everyday performance to be about the same or a little better because you’re then free of the slowdowns from the mechanical part of a worn fusion drive.
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Do you mean the Luna Display? If so, been there still doing that on an occasional basis. Fast 24GB M4 mini running headless (no physical monitor attached), various display clients including 2014 and 2019 27” iMacs, as well as an occasional iPad.
It’s a workable solution for quick use, but inconsistent when used for long stretches. Connecting the two via Thunderbolt or Ethernet is much better than WiFi because the latency with WiFi creates a pretty laggy experience. The biggest challenge I’ve found running the host Mac without a physical monitor attached is that it”s hard to wake up. So there’s the added step of using a VNC app (I like Edovia’s Screens, but they’re kind of all the same) to wake it up before starting the Luna Display session. At its best, hard-wired latency is excellent at 2-3ms, however it will occasionally spike much higher and the display output gets laggy and pixelated for a few seconds.
Was it worth the $80 it cost? I think so as it’s handy for quick access to my GarageSale. Does it provide a perfect, highly stable 5K monitor experience that I would want to use for hours at a time? No, but only because of the maybe 2-3% of the time that it suddenly has high latency and pixelation.
Will that work with a broken fusion drive that won’t boot up?
You should be able to boot into Recovery mode, that will let you download MacOS via internet and install to an external SSD. You may not be able to get the current version of your MacOs that way, but once it is installed to your SSD and you’ve selected it as your startup disk, then you can go through software update to bring it up to the current MacOS version.
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Is a SSD with a Usb connection fast enough or do I need a faster connection to make a bootable start up disk?
A modern USB 3 drive is more than adequate, as the USB 3.0 port speed of a 2015 iMac will be the limiting factor for bandwidth. You could improve performance (20Gbps vs 5Gbpsj by using a Thunderbolt enclosure, but the cost is much higher and likely not worth it on an iMac that only has Thunderbolt 2.
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This is what I was talking about and it looks like 2015 is not covered, but for older iMacs, it is super easy: Use your iMac as a display with target display mode - Apple Support Just plug a cable in, hold some buttons down, and boom, your iMac is a display.
But as there are custom boards you can install inside the iMac to support this behavior if your iMac doesn’t support native target display mode; the install is more difficult [glass on iMacs is very easy to remove once you’ve done it and gotten over the fear - it’s the edge tape and stuff that gets annoying]. The biggest issue is the quality of these aftermarket boards and how they get installed. Ya might need to do some soldering!
Thanks MTB_Matt for the great advice. I now have a second working 2015 iMac that I am using as a backup machine! Apple should have told me this was possible when they declined repair when the internal fusion drive failed. The Recovery mode could not make the bootable external SSD disk drive but my working computer did. I am now moving into the syncing world so we will see how that works. I have heard there are issues getting it the first time.
Video posted today is exactly what I was talking about, in case you feel adventurous https://www.youtube.com/watch?v=YQozfHU2kjA
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Interesting. I am currently running the 2015 iMac with a blown non-functioning fusion drive with an external SSD card. It is actually 30 seconds faster than an original fusion drive at startup. A power outage with the two 2015’s sitting side by side shows this.
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