The latest version of Blender requires at least an Intel Skylake integrated GPU. Even if you could upgrade the CPU, jumping from Haswell (4th gen Intel) to Skylake (6th gen Intel) would not be possible.
Your only option would be an eGPU connected via Thunderbolt.
Honestly, you’d probably be better off finding a used Mac that meets the requirements and selling the 2014. You can find 2018 Mac minis for next to nothing and the GPU is supported. You can also upgrade the RAM in the 2018 to 32 or 64GB for relatively cheap.
If you want to go the eGPU route, your best bet would probably be to find one the AMD RX 560 4GB pucks from Sonnet. Anything else is likely to be more expensive than just starting with a fully compatible Mac.