Hello @laurent,
Stepping in as, being AFK for a week, I gave this another shot... And got pretty nice result! 
Using the Markdownr app on android does a nice job converting web pages to MD with nice front matter and summary options, and automatically pops up the share panel again to successfully import into Joplin mobile. Main "issue" is that no choice is given for the target folder. If that choice could be given and assets downloaded at import time, that would give us a pretty good capture solution.
If there was a more complete solution to build upon this, the Zettel Notes sharing options are surely a good inspiration as the convertion to md seems to work very well, it offers the choice of target dir (or note), and has additional sharing targets like adding to note, capturing link only... Making the closest thing to our beloved web clipper experience.
Hope it helps
J