Getting your microscope where you want it, will be a big d.i.y. project. and probably not worth the effort. Your limitations are based on the N.A. and degree of colour correction of the objectives, the precision and design of your sample preparations and the objective contrast plus the contrast techniques employed. I would also add the condenser colour correction and N.A. and illumination intensity. Yeast and bacteria are notoriously difficult to image with precision but obviously it can be done with a light microscope, otherwise photos and drawings from the pre electron microscope era would not exist.
Common phase contrast is only partially useful for small subjects, such as the ones you are targeting. It seems that the default phase system that is made by microscope companies is dark phase and most likely dark medium phase. While this is a good general purpose phase type, it has an undesirable degree of halo and shade off, which is highly detractive when viewing small subjects with important inclusions. Bright phase is a better system for yeast and bacteria, as is anoptral and other proprietary types such as B -Minus. B-Minus is particularly good for imaging small inclusions, such as those found in many bacteria and yeast pores. The phase halo, is not so objectionable with it as is the case with anoptral phase. Apodized phase controls the halo to a large degree. Read this link.
https://www.microscopyu.com/tutorials/s ... -artifacts.
Since your microscope has 160mm fixed tube optics, the possibility to d.i.y. an improved phase system on your existing stand does exist, since there were multiple manufacturers of alternate phase systems using the 160mm system and the parts are often not that expensive. It takes time to find complete systems, though.
Dark field resolution is limited by the quality of the condenser employed, the quality of the high magnification objective(s) and the illumination capacity of the microscope stand. You will not be able to adapt your microscope to an acceptable level of dark field performance in order to image such small subjects, without again getting into more diy'ing, this time upping the illumination by a considerable level, buying into a fine condenser and at least one high resolution objective. You will need either a Paraboloid DF condenser or preferably a Cardioid DF condenser. Cardioids are mirror condensers and are free of ca, something necessary for high resolution DF. You will also need an iris diaphragm equipped 100X objective lens, preferably with an N.A. of 1.30 or higher. This usually implies either a fluorite or apochromat type.
It sounds like you prefer to keep your glasses on while at the microscope, so with that in mind, you need to find an acceptable pair of high eye relief eyepieces. These are designated by having a little icon of a pair of eye glasses on them. I believe that the Chinese make such eyepieces but they may be for the infinity corrected systems. I see them marketed as replacements for Bausch & Lomb , probably similar to the ones on a Balplan, which have very high eye relief.
For your purposes, if I were you, I would use my microscope as is and begin the process of assembling a more finely tuned system based on a second hand stand. Once you get enough of it put together, you could sell your existing microscope. Normally, such a microscope would bring very little but with phase, it should have some resale value. Both Nikon and AO had quite extensive phase systems; dark in several contrast grades, bright in the same. These show up on the used market. Something like an Optiphot or a Labophot, can be customized as to illumination output and parts for phase are fairly well represented on the used market. There were dark and bright in various contrasts and I think apodized too? Someone else might know.
With AO the picture is a little different. AO microscope stands were made with specific illumination capacity. AO series 20, 120 and 420 infinity corrected microscopes have rear mounted 100 watt illuminators , can use Cardioid oil DF condensers, as well as Toric Cardioid oil DF condensers, which have a wider illumination field, so can be used with lower power objectives. Both are very good for high resolution DF. The phase systems are inexpensive and the eyepieces are high eyepoint. The pre-infinity series 2/4 system had a broad array of phase optics, 26 objectives in total but by the time the infinity system came along they had reduced the number but still had bright ,dark and B-Minus, so enough to cover the basis for microbiology.
Ultimately, to image such small subjects accurately, you will need to refine your sample preparation as well. That means no shortcuts as to the quality of slides, coverslips and strict adherence to your sample thickness and adjustment of your illumination beam. Higher grade microscopes also offer better options for modest filtering of the illumination, creating better contrast conditions and the optics usually have superior contrast over what average grade Chinese optics provide.