What could be helpful is if there is another way to mechanically trigger the shutter on the r5ii with a cable that goes to another input on the R5ii other than the shutter release in the front. I doubt there is but figured I'd ask.
p.1 #2 · Canon R5ii shutter release other than front of camera
I'm not quite sure what you are trying to do, but there is only one wired remote release on the R5 II. The shutter is electromagnetic, not mechanical. With proper circuits you could wire the camera and flashes to trigger together, but the flash exposure at typical shutter speeds would be missed as the shutter opening will be later. You would need to slightly delay the flash compared to the shutter, which is why we have flash trigger controlled by the camera for nearly a century. If you are not adept at circuits I will give you a hint that optoisolators are your friend.
p.1 #3 · Canon R5ii shutter release other than front of camera
I don't use R-series cameras. At one time Canon manufactured EX/EXII flash units that acted as a remote sensor, and fired the shutter (pretty sure it was 270EX or 90EX). Canon also manufactured small (quarter-sized) wireless remote triggers that fired the shutter. Last but not least, canon makes software you can use on your computer or you phone to remotely fire the shutter. With the software, you have complete control over almost everything the camera can do.
p.1 #4 · Canon R5ii shutter release other than front of camera
Like EB-1 says, I'm not quite sure what you are trying to accomplish here but I'll throw this idea out for your consideration.
Would something like the Canon BR-E1 remote shutter release work? I use one regularly and it works quite well. You just have to change the drive mode in the camera's menu otherwise there will be a 10 second delay from the time you press the button on the shutter release and the time the actual shutter is activated. The manual has the instructions on how to set it properly.
The remote shutter release will activate the shutter only. Any further activation of flash units etc will need an additional activation device.
p.1 #5 · Canon R5ii shutter release other than front of camera
There's software that allows you to trigger the camera via USB-C. I have "Shutter" on my iPhone that I can connect to the camera and operate "remotely." Straight USB-C to USB-C. Has bugs but works. Try for free, then reasonable price.
