What are you primarily using the machine for? If you're using it for work like photography and video work such as Affinity Photo/designer/publisher or Adobe CC, those are long in the tooth, to the point they're beginning to show their age. You may consider an upgrade, but a better SSD (Apple OEM or a compatible SSD that doesn't need to be adapted) [link|https://www.transcend-info.com/product/internal-ssd/jetdrive-820|like|new_window=true] [link|https://eshop.macsales.com/shop/ssd/owc/mac-pro/2013|this|new_window=true] may help you get a bit more life out of it, but it may not be much.
If you use it for less intensive work, slap an SSD in it, and you'll easily get a few more years out of it. There is no need to upgrade unless you want a machine that isn't a pig in terms of power consumption, given that the Xeon processors were always known for being heavy on power consumption—sometimes as much as a laser printer in a dual Xeon setup with a professional grade GPU like a Radeon Pro/FirePro or Quadro!