I am glad you are looking to use the Openflexure microscope. If you are starting with printing new parts, I would recommend the version 7 design A new alpha! v7.0.0-alpha2 - Announcements - OpenFlexure Forum. It is still labelled as an Alpha release, but we have now built several and there are no real issues in the hardware. Most of the instructions are a lot better than the V6 release on the main website. If you have any problems you can always drop in here for advice!
Your previous experience has suggested that a 40x objective lens is needed. You may be able to get away with the Raspberry Pi lens, which has a roughly equivalent field of view to a 20x objective, but does have remarkably good resolution. It is quick to try anyway, you just need to print the low cost optics part. @nanocastro has posted in this forum on soil microscopy, with a nice guide linked from that post.
Note also that the Openflexure microscope is an ‘inverted’ design. This means that the lens is below the stage, so you will need to use a coverslip and turn the slide over to get a high magnification lens close enough to the sample. ( 40x Objective Can’t Focus - Request Help - OpenFlexure Forum. There is an Upright configuration in V7 ( OpenFlexure Microscope - V7 upright - instructions)