org-remark VS olivetti

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 also use writeroom because I prefer its simplicity, but worth mentioning that olivetti is the more popular writing mode.

There are nice looking distraction-free setups with https://github.com/joostkremers/writeroom-mode or https://github.com/rnkn/olivetti or https://github.com/joaotavora/darkroom or https://github.com/rougier/nano-emacs

For batteries-included packages that give you dedicated prose-writing modes, see also olivetti and writeroom-mode.

You could use olivetti-mode or Writeroom mode.

Another option is the Olivetti package. https://github.com/rnkn/olivetti

Don't have a specific tutorial except to suggest setting org-mode to use olivetti and a nice variable pitch font for writing prose. You might also like the poet theme.

If you want to try out some third-party packages in the beginning, I think the most "bang-for-your-buck" you'll get is with Avy. Also, expand-region. And specifically for writing: olivetti-mode, flyspell, dictionary, and Nicolas Rougier's Nano and Elegant Emacs setups.