Interesting. Never heard of this, there are so many of these systems now 
. Zotero is for the organisation of academic literature. It also works well with most websites. But it is not at all a service like Readwise. There are packages for spaced repetition in Emacs but there is probably no working solution for Kindle excerpts.
Emacs and orgmode are fantastic and provide ultimate freedom to tinker and create your own system. But this way requires an investment in time and effort. And there is a scary learning curve. Given the details you provided so far, I would advise to use the systems/software you are already familiar with or to chose Obsidian. The search for the right tool only takes away time that you could spend on your topic. 
On this see also: Why not use Obsidian and/or Logseq instead of Emacs Org-Roam+Org-Mode? - #10 by laotang
Edit: Just watched the first eight minutes of August Bradley’s intro video. I don’t know how to better phrase this but his system feels like a distilled version of all pkm systems of the last few years. P.A.R.A., second brain, getting things done etc. Almost as if ChatGPT had put together something. Just an observation, if this works for you then please, by all means, continue.