@Gxzn
The hardware on the Verizon model isn't the best match,although useable:
http://willmyphonework.net/
Even if you could convert the phone to run the full T-Mobile Galaxy S6 Stock ROM,that will not change anything from what you currently have,the hardware simply will not support it as a 100% T-Mobile device.
All you can do as compatibility goes is drop in the T-Mobile SIM & Set up the APN for T-Mobile & use the bands available.
You may also need to contact VZW,IDK,they're a different animal altogether vs GSM carriers (most of the rest of the world & AT&T/T-Mobile)..........................