As one may guess from the photos above, the polyamide endplates did not work well at all (and that is an understatement). So I had to switch back to aluminium, as it is still rather inexpensive and in the end I was not sure if even Delrin would be rigid enough in this case. As a matter of fact, Alcofribas had suggested that I use brass instead. I might have taken that route if I did not already have some alu in hand. Brass seems to be more expensive and quite a bit heavier than aluminium, but apparently it is machined dry. My setup is rather poor in handling the coolant generally required when milling aluminium, so this would have been an important advantage.
Having to go back to aluminium motivated me to do some research in milling speeds and feeds and coolant types. I found out the MF70 spindle was powerful enough to take deeper cuts in aluminium (1.5mm vs. 0.5mm) at my current feeds (about 90mm/m and 8000rpm). Apparently these numbers are still far from optimal, but they were a big improvement over my previous setup, which took far too long to mill the parts and caused the spindle to overheat pretty quickly. If the MF70 spindle horsepower permits, I plan to even further increase feeds (once I upgrade the steppers) to see how far I can take this. I can also confirm that ethanol performs quite well as a coolant for milling aluminium.
Another “improvement” was that I finally managed to obtain a dial test indicator, thanks to ebay. Measuring the backlash on all axes, I found it to be around 0.1mm, which is not exactly low, but easily compensated in software. In fact, this performs so well that I may not even bother to implement the split-nut anti-backlash nut design I was referring to in my previous post.
Finally, even though the new parts came out quite nice, the performance improvement with the old steppers was marginal. So, I decided to give priority to upgrading the steppers and the drivers over upgrading the mechanical parts. Depending on how that goes, I will decide if it is worth to actually rebuild the Y and Z axes.