Continuing the discussion from LANC control of Atomos - Using Arduino as Master to trigger recording:
I've been working on the same problem and your code was a massive help to me and my clanker. With some AI help, I was able to prune out some of the REC-LA emulation stuff that slows it down. This functions basically as a Sony camera emulator - triggering the record function by flipping the bit that signals that the master is recording.
I went down the same path as you at first thinking I needed transistors and whatnot, but at least for 5V logic, you can just wire the output pin directly to the jack tip otherwise a series resistor might be warranted. I 3D printed a little domino-sized enclosure for a DFROBOT Beetle, a female 2.5mm jack, and room for components, but now i can make it smaller. I'll link to that on my github once I'm done revising the design.