The paper has a tungsten balance if shot in-camera and an ISO of about 25 is a good starting point. I think after filtration, you will probably be at about ISO 6.
Your process looks ok, but as you say, a low contrast first developer may help. I have found adding 1 g/l - 5 g/l of Sodium Sulfite to the color developer helps reduce contrast. Be careful though, it can reduce contrast and dmax very rapidly and lead to red blacks.
A contrast mask is probably the best route, but I have not worked with it for months due to the emulsion work.
PE