org-roam-bibtex VS md-roam

In addition to org-roam, I would recommend checking out https://github.com/org-roam/org-roam-bibtex for attaching notes to specific papers that are indexed by an application like Zotero. Some good tutorials for it are here: https://rgoswami.me/posts/org-note-workflow/ and https://blog.jethro.dev/posts/how_to_take_smart_notes_org/ and https://emacsconf.org/2020/talks/17/ . Since current org-roam (post-v2) uses the normal org-mode "id" form, you can make any "headline" into an roam-registered node. You could then split your long derivations into different headlines, where the body of the headline or descriptive text could refer to prior nodes, but any latex is inserted using org latex blocks https://orgmode.org/worg/org-contrib/babel/languages/ob-doc-LaTeX.html . You've maybe already considered this (and I wouldn't be surprised if local references were easier to do in straight latex), but org-roam-bibtex is really nice for interfacing with your citation system for other papers.

I think you should also look org-roam and org-roam-bibtex (I can't configure org-roam-bibtex properly because I'm on Windows).

Taking this one step further, if you use the excellent org-roam package (which if you don't you should definitely check it out, its great for note-taking), there is a package called org-roam-bibtex which communicates very well with the aforementioned packages. With it, you can create bibliographic notes as part of your org-roam repository from their bibtex entry and initialize the file to also include a link to the pdf from org-noter. Once you learn it, its a very powerful and streamlined workflow for working with articles which makes your life very easy. Its quite a bit of reading to do on your end on how to make all these work together, but trust me its worth it.

Have you followed the discussion here: https://github.com/org-roam/org-roam-bibtex/issues/159

If you want to use it primarily for literature notes, maybe you should have a look at the current state of https://github.com/org-roam/org-roam-bibtex/tree/org-roam-v2 before deciding.

I was also taking a look at https://github.com/nobiot/md-roam to make org-roam work with markdowns

Maybe nobiot/md-roam: Use Org-roam with markdown files by adding Md-roam as a plug-in. Mix and match org and markdown files.?

Could something like org-roam be implemented without external database? Yes and I think that that few projects were already shared here on /r/emacs last years doing exactly that. The thing is that on the same amount of data it will be slower then org-roam (I don't have benchmarks, just my assumption based on my experience) and using markdown files instead of org mode files (md-roam) would not be possible.