Personally, I think its a great idea. The labour cost seems to be quite high, however, if you're happy with paying that rather than DIYing it, then I'd say upgrade the SSD.
One thing I will say is that SATA 3.0 has theoretical speeds of up to 6Gb/s, and I'm presuming you want to upgrade the SATA drive. I'd personally recommend buying an NVME M.2 drive and sticking it in the M.2 slot on the motherboard (I'm calling it M.2 because idk what its actually called). You'd need an adapter that can be found fairly cheaply on eBay, and it wouldn't be branded, but you would see far better speeds than you would with a SATA SSD. I'm talking 2400 Megabytes per second rather than 600 Megabytes per second (or 6Gb/s), as I believe the slot is limited to 2400 or something like that.
This would also allow you to keep the Hard Drive in there as some mass storage that you could offload files to if you wanted.
If speed matters to you, or you would like to keep the SATA hard drive, I'd recommend going for this option instead, as you get the far faster speeds, but remember: YOU WILL NEED THE ADAPTER
I believe its just called an Apple Blade SSD to M.2 drive, and from there you should be able to use an M.2 drive normally. Luke Miani has a video on upgrading a 2013 iMac (different year but I believe it should have the same slot), and that gives you a rundown of how this kind of process is carried out. Just be careful, as there might be adapters for the 2012 Retina MacBook Pros (and in turn the 2012 iMac 27"), which uses a different slot (some form of mSATA I think). The slot in the 2017 uses the same slot as the Late 2013-Mid 2015 MacBook Pros, except the 2017 runs it at a faster speed.
Hope this helps
P.S sorry if you already were meaning doing the Blade SSD swap because if you were I've just wasted both of our times
