org-remark VS md-roam

I use something similar, called org-remark. By the description of both, they seem to share functionalities. Just mentioning it for visibility :)

org-remark hits some of your requests, but I think lacks the git parts in point 3.

https://github.com/nobiot/org-remark might also work

There is also Org-remark, which lets you highlight and annotate any text file using Org-mode.

Also if you're annotating plain text, org-remark is great. I love not having to use the mouse to select text.

What happens when line number changes? For example https://github.com/nobiot/org-remark handles this changes, but can only anotate org-mode files

Org-Remark, for plain text files perhaps.

you might want to try out org remark

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.