It's pretty trivial to keep an open "project.org" buffer open for whatever project I'm working on. This is pretty natural in emacs but sometimes vim users aren't as confident with tabs or buffers.
Which gives you "full" orgmode in comment sections of your normal source code (haven't actually tried it myself).
Overall I think you are on the right track with your goals but, yes, orgmode does a great job of solving them already with a bunch of additional features too.
First of all, the buffer will be too small for all tasks created on a branch and you don't want to have this buffer big, unless you have 2 monitors and have Emacs frame with this buffer opened for a half of it.
Also this process is not automatic - you have to navigate to the buffer and close the task when it's done. Also from the that buffer it's impossible to sync outside (GitLab/GitHub) or share your tasks with a team, what is possible with Kanboard if it's running on DigitalOcean/Heroku or simply synchronizes with GitHub.
If one really wanted to do it all in emacs' org-mode but also have it be automated and sycned with the code, I believe the solution would resemble something along the following lines:
Literate Programming in the Knuth sense with org-mode file using tangle[0] blocks holding the source code which can be used to generate and build the project, and a combination of regular emacs lisp code and some org-mode evaluated emacs lisp[1] to do whatever automation is needed.
Now the TODOs can be in the org file, which contains the source, and may also have a reference (automated) to the TODO, which when closed with the regular org-mode command (and is generally only two key chords) will remove the comment as well as update your agenda, calendar, kanban, etc.
There is also https://github.com/alphapapa/outshine
Which gives you "full" orgmode in comment sections of your normal source code (haven't actually tried it myself).
Overall I think you are on the right track with your goals but, yes, orgmode does a great job of solving them already with a bunch of additional features too.