Since you're taking photo instead of video, try turn on "iZoom", you can zoom up to 150mm, the quality should be quite good.
I just receive my Aquako IV diopter which is stronger then the SMC. It's suffering from vignette issue just like the Subsee+10. I'm working on a solution to make that lens closer to the port, so if it works, I will share it here. Looking at the diopter flip holder, they could have put the circle behind the tab and hinge so the 67mm thread is closer to the front of the port. Don't know why they design the way it is now.