Thanks for your reply, William. Yes, now that you say it it is quite obvious that the light source has to be bigger but with a narrow lens for the low magnifying lenses. Tested it on my computer monitor, nice and even image. Was not able to resolve the issue with the low working distance tough, I guess it is a problem of the objectives themselves. If anybody knows where to find reliable reasonably priced 1x and 2x objectives please let me know.
Yeah true, the motion speed is rather slow, this is why I made faster gearing. For my upcoming project I don’t need the moving stage at all and actually a simpler microscopy imaging setup would be sufficient. However, as the Openflexure microscopes are fun and easy to adapt, I thought I’d use those 
Talking about adoption, I will use reflected light microscopy and I made a led ring that holds 9x 5mm LEDs around the objective of the upright microscope:
upright_LED_mount.stl (200.6 KB)
Do you think it would be worth considering an epi- illumination lens setup to achieve a more uniform illumination? I.e. the epifluorescence optics with just the beam splitter and without the color filters, and would that even work?