With a slanted back tub, do you think that 520mm is too deep since we will already be reclining?
About the slanted back, the gradient is actually very subtle: the slant is more of an esthetic feature.
Consider that in a deep soaking bathtub, the lungs are fully submerged, so the body becomes lighter (lifted by the hydrostatic force) compared to a traditional bathtub.
This means that you can keep the back at an angle (and adjust the depth of the immersion) independently from the fact that the back is slanted or not.
I think 520 is a good depth and is still quite comfortable to step in and out.
Said that the size balance inside the ofuro is a very personal factor and we recommend to our customers to create a mockup (using cardboard boxes, or maybe the space between the back of the sofa and a wall etc.) to simulate the space inside the ofuro and imagine sitting in it.
Also, I suggest also making the calculation of the number of gallons of hot water required to fill it up and consulting with your plumber to verify the compatibility with your HW system.