Fusion Drive has died. What was the best recommended replacement?

Your SSD is much smaller than your HDD. I’m going to guess your HDD is failing.

Replacing it with a new drive. I would recommend going with a hybrid drive (SSHD) like a 2.5” Seagate FireCuda. Or, if you want go with a 2.5” SSD but its going to be more expensive than a HDD or a SSDH drive Mac mini Late 2014のハードディスクドライブの交換

You’ll first need to break the Fusion Drive set following this guide first How to split up a Fusion Drive.

I would remove the blade SSD as its so small its quite useless. If you want some real performance you’ll want to upgrade it to a larger drive and make it your boot drive. Here’s a good reference on these drives The Ultimate Guide to Apple’s Proprietary SSDs and here’s the guide Mac mini Late 2014 SSDの交換

Using a 128 GB or 256 GB blade drive will offer a huge perforce gain and then you can have the HDD for your deep storage. This is the best long config gaining the best of a dual drive (not a Fusion Drive).