Wow, I didn’t realize that my analysis and question became so long 
. Props for anyone actually reading through that.
After some further digging I realized that I totally misunderstood the rotation_distance setting. I didn’t realize it affected steps/mm, and therefore affecting the feedrate as well. In my case it would be ideal to change the rotation_distance on the fly, but as far as I know that’s not possible (Something similar to Marlin's M92 command - #17 by dmbutyugin). But I think I can manage with just using a rotation_distance that corresponds with the average circumference, and post process my gcode to compensate for the different angular distances and velocity.
Next step is to actually implement this system on an ender 3 with a z belt drive modification. Because I realize now that the current z-axis, consisting of a lead screw and a rotation distance of 4, really is not up to the task of non planar printing. Aside from the erroneous rotation ditance on the y-axis, the z axis is most likely a major bottleneck as well, as the steps/mm on this axis is huge, ausing the printer to slow down when a z movement is initiated (which is ususally right before the corners…)