Best practice when summarizing (not reviewing) research literature I recently started my PhD program and have already accumulated a vast amount of papers on my to-read-list. I never had troubles reading and remembering content of academic papers, but this pile is giving me headaches. So much so, that the comment and note functions of my reference management software (Zotero) does not seem to be sufficient to keep track of all the information.

Previous attempts to structure key findings in all this literature have resulted in fragmented lists that just add on to my confusion. I also tried mind mapping applications but feel like there must be some kind of best practice on how researchers structure and extract information on a larger scale (~150 papers).

EDIT: My goal is to quickly gain an extensive overview of my current field of research and its neighboring constructs, including unanswered or potential research questions, hypotheses that were previously investigated. What tools are commonly used beyond reference management software?

I'm aware that this questions has no objective true answer, but find it nonetheless important. Thank you!
I would let your specific publication needs (from your work) and researches drive the literature research.  Don't try to get a big overview of all research.  Once you have done a Ph.D., you will be the world's expert in your thesis and have reasonable expertise in a growing circle around it.  But don't expect to have encyclopedic knowledge of a whole subspecialty.  Even older faculty have their "spikes" based on their actual research careers.  
Don't take so many notes and don't use a note taking system.  Instead use a filing cabinet and paper copies of publications.  Sort them into some category scheme (does not need to be MECE).  If you take notes on a particular paper, just do it in pen on the paper or on looseleaf that you attach.  At the end of the day, don't try to have some perfect retrieval and synthesis system.  Instead read for what you need at the time and then file the paper in your file cabinet.  Come back to it when needed.

[Note that second readings of papers often give different insights.  You are a different person (more sophisticated, better able to extract things) a couple years later into your Ph.D.  Also, you may be looking for different things later.  Don't expect or try for perfect comprehension the first time.]
Occasionally get the references to the refence (all or many of the citations for a journal aticle).  Don't do it all the time and draw the line at some point of moving backwards.  The point is to just get some feel for types of papers (datapoint versus review, technique versus results, etc.)  
Become familiar with some encyclopedic references.  For instance in solid state chemistry the following are very valuable:  Wells Structural Inorganic, West Solid State Chemistry, Greenwood and Earnshaw Chemistry of the Elements, Phase Diagram for Ceramacists.  Consider to buy some or get your PI to do so.  Or at least find out wher held in library (ask for them to be pit on reserve).  Note, you won't read them exhaustively.  But you'll likely do more than just targeted searches.  Occasionally wander down into the Mandelbrot.
While SE likes some generality and you will get responses from different fields, it is helpful if you can give a little specifics also.  That way you may get both general and specific advice.  I can't tell if you are doing history or psych or business or physics.  Experimenter or theorist.  Little bit of info/perspective on your situation also helpful (2nd year student in 4 year Ph.D. or the like). Do this in a light fashion so you don't get accused of a too personal question.  And so more general responses are welcome and helpful to future readers.  But still...give some more info so better targeted advice possible.