You will not get an official answer from T-Mobile. Not sure if I have seen any custom roms but if you want to look the best place to search out would be xda developers website. Note (no pun intended lol) that it would require rooting your device, any issues you have with it or service will not be supported any longer, any T-Mobile specific features like Wi-Fi calling probably will be gone not to mention if you don't know what your doing when you root and load a custom ROM you could easily brick your phone. But I as said the best place to look would be that website and see if anyone was able to custom build a ROM if that device could even support it.
Thanks Steve, that's the answer I needed after exhausting all other
sources. Like with computers, it's time to upgrade the phone. Thanks again