Performance increase from Fusion Drive
I would say: No. 

My experience of using a Fusion Drive in a 2012 Mini for several years is that the shared performance benefit lessens as the drive fills up. 

Secondly, and perhaps most importantly: a Fusion Drive has two points of failure, and a nine-year-old hard drive is increasingly likely to fail, at which point you will have only your backup. ;-)

Your best course of action is to use the 256Gb SSD as your System disk and farm out user data 'overspill' to a faster external drive.