Gutenberg custom blocks

Wow, though my question was removed. Someone edited the title without any warning…

I finally found some (dirty) ways to manage what I wanted to do.

So for my first question, I came accross this way : https://florianbrinkmann.com/en/display-specific-gutenberg-blocks-of-a-post-outside-of-the-post-content-in-the-theme-5620/ which will enable you to play with the content output. I splitted the render of the block depending on the blocks name, so I can render two or more outputs (as string) in different containers of my page template. But obviously be careful with this way if you’ve other extensions that already manipulate the content or their filters.

And for the second one, I finally changed the default content directly using the default_content filter.
Fortunately this last one work with blocks, but don’t forget to check if the current editor mode is well Gutenberg. CF : https://wordpress.stackexchange.com/questions/348426/how-can-default-content-filter-tell-if-content-will-load-in-block-editor-or-clas then you can pass a block as a comment in a string (as it is written in your database).

Hope it will help.