I somehow missed the whole "MFJ TNC" part.? Frankly in this day and age you can do much better. Sure, the TNC will work to an extent but it's a Z80 based hardware modem that has long been surpassed by software and DSP modems. It also limits you to 300bd AX 25 whereas a software or DSP modems can do other modes such as IL2P or Robust. Even VARA if you really must.
I realize that you are getting conflicting advice in this email group but for my money and given the hardware available to you I would be doing a software modem. Win10 is a capable platform for software based modems and a sound card interface is trivial if you don't already have one. Keep the MFJ for local FM packet on 2M.
Feel free to drop me a line off list me. My call at the League finds me. There's more than one way to skin this cat.