I can vouch that Grbl v1.1 works great with JTech lasers. I’ve got their 2.8W version. I also have their 4W, but have not used it yet. The main difference is the 4W has a slightly larger spot size, which kind of negates the increased power and cuts a bigger kerf.
But, it’s connected through an older Carbide Motion board, where I had to solder the laser signal/ground into the set of large pads on the side of it. At the time, Carbide used this same old board in their Nomads and connected these pads to a daughter board to power/control the Nomad spindle. The SO3 board has changed a lot, but it should have a similar set of pads there. Anyhow, if I recall correctly, I had carefully map which of these pads were the signal, power, and ground. The power pad sent the full 24V motor voltage. Make sure you figure these out and label these clearly to avoid any shorts or damaging your electronics.