@mayer - Sorry didn’t see your post ;-{
None of the newer 2012 and up have MXM boards so no its not upgradable. The Video RAM I guess could be upgraded to 2 GB but that would require desoldering the BGA chips and having access to the needed replacements.
Frankly, I think thats pushing things a bit far. I would look at an external GPU unit, you who'll still need the GPU board (PCIe). Due to the limits of TB2 you’ll need to use the eGPU port fort the display output.
'''References:'''
* [https://www.macworld.co.uk/how-to/egpu-mac-3690094/|How to use an eGPU with a Mac, including older models]
* [https://egpu.io/best-egpu-buyers-guide/#tb2-enclosures|Best eGPU Enclosures]