Doesn't the MiSTer core/emu already offer auto-fire from the config? Retroarch has a function to set a button to auto-fire on/off but that would not help you when using a PCE/TG mini pad since no extra buttons are present. (I guess that's why you asked)
While MiSTer has the autofire, it's not exactly accurate to the original hardware of TGX-16 - in fact it tends to even drop frames during autofire, making it inconsistent. MyLifeInGaming's video on MiSTer FPGA's TGX16-core documents this on
here.. Probably being a perfectionist purist here, but I've been curious of the Turbo Pad to join next to my other gamepads closing as much to the original controllers as possible.
Retroarch's Beetle PCE looks to be pretty accurate but I've moved away from using RetroArch a long time ago due being kind of cumbersome to use with having to adjust input latency options among other things more than actually sitting down to play the games. I might explore the controller there though.
But yeah, mainly just wanting to get this controller for not only TGX-16-games but possibly try other systems with 2-button configurations like NES and Sega Master System on both MiSTer and Retroarch.