It says the board will do 5v or 12v (you adjust the pin jumpers)You probably don't need opto-isolated kit. I think that board is limited to 5V on the control side, and is active-low (pull the input to ground to enable the relay), although they're changeover relays so probably not a massive problem.
I would be looking at a basic trailer bypass relay - designed for the job!
I was thinking in a small project box.I don't see any issues with you using this board, I did something similar on a trailer I had some time ago. This is pretty similar to many power distribution boards available for accessories etc. The one thing I would do if you use this is to make sure it is well protected from moisture. A lot of these types of relay boards are not coated. A board made for a vehicle is usually coated to prevent corrosion issues. Just a thought.
I used a sealed trailer wiring and relay harness from Kuryakyn on my ST1300. It is set-up for separate brake and turn signal lights, so it has the five wires that you need. It includes the trigger relay that isolates it from the battery when the motorcycle is turned off.Most of the trailer bypass systems I see are for a 4 wire system, and I'm running a 5 wire setup. (running, brake, left turn, right turn, ground)