Hello to all,
I've evolved my GTD workflow a bit with Joplin and I wanted to share it with you if it can be useful.
The general principle is to use notebooks to classify todo's because it is easy with Joplin to move todo's from the list to the notebooks.
Then, things are organized in the following way.
To separate domains that are impermeable to each other I use the concept of "area". The idea is very simple. It is the first filter of my todo. I have a "(a) pro" tag and a "(b) perso" tag to separate what is professional and what is personal. I don't want to see my professional todo's when I'm doing my homework.
The second aspect concerns projects. For me projects are represented by a note in the format "(P) project name". Since we are in the GTD philosophy, the idea is that a project is a thing to be done and not a category of todo classification. The note I create for each project is a gateway to the project. I note general information about the project, I put links to other notes, follow-up tables, etc...
Nevertheless these projects belong to domains of responsibility that allow me to organize my documentation. So I have another notebook called "01 - Domains" in which I have 4 sub-notebooks corresponding to the 4 main areas of activity that are mine. It is in these 4 notebooks that I put my project notes. It looks like this...
- 01 - Domains (notebook)
-- Domain 1 (subnotebook)
--- (P) Project 1 (note)
--- (P) Project 2 (note)
--- Other note with meeting note (note)
--- Other note with table (note)
--- Other note with file (pdf, xls, etc) (note)
-- Domain 2 (subnotebook)
-- Domain 3 (subnotebook)
-- Domain 4 (subnotebook)
This way, I use Joplin to manage my projects and their documentation. But how do I connect my GTD notebook and my Domains notebook? With links of course 
The creation of a todo is basic. Thanks to the template plugin I quickly create a task (either pro or personal) which goes in the "INBOX" folder of my GTD notebook. The template looks like this for "pro" todo...
If I create a test todo that belongs to a test project, I get this.
When I review my INBOX notebook I usually add time estimates for todo with tags ("5m" "10m" "30m" "1h" etc) and context ("@home" "@work" "@transport"). Once the todo has all the necessary information and I think it can be activated, I move it to the NEXT notebook. You get the idea.
Now comes the most important part. Find a way to visualize what you have to do so you don't forget anything and in a pleasant way. This is where the power of Joplin's ecosystem of plugins comes in.
Thanks to overview plugin and embed search plugin I can have an overview of all the todo's according to their status in the GTD notebook. It looks like this.
The overview plugin produces very beautiful but static tables. If you want to be able to check todo's without leaving the document you can embed the same search under the table and integrate it in a spoiler (to avoid polluting the view).
Also, since projects are notes, from the project note you can see the notes and todo's related to that project in the backlinks plugin. You will notice that the emoji embedded ( 
) in the todo template title is useful here to distinguish between todo backlinks and note backlinks.
This is how it works quickly. And it works well