I received an answer almost right away!!!
The advice was to focus not in the middle of the image but 1/3 on the outside as sketched on this image:
I didn't really know how to achieve this (neither TSX nor SGPro allow to focus "off center")
So, instead I focused as normal and then move the focuser in and out within the critical focus zone (and a little beyond) to see if the curvature changes.
The critical focus zone of the TOA-130 with the Super Reducer is 170 microns. The step size of the FLI Atlas focuser is 0.085 micron / step. I.e. I need to move the focuser 2000+ steps to go in and out of the critical focus zone.
Unfortunately, this didn't make large (enough) difference. The curvature was always between 17 and 22 degree. But never significantly better...
The answer from Takahashi Europe was somewhat disappointing:
"The field is flat with the 67FL.
The field is slightly curved with the SRD645."
Here are the spot diagrams for both:
|.67 Flattener||645 Reducer|