Eventually, there's an agent installed because I have now a log in /var/log/m_agent.log.
I have upload one of my custom PKG and also a genuine Firefox DMG.
On the console side, Meraki still says that "Client synchronization is not yet complete."
In the Installed Apps, I can now see an entry :
Dec 30, 2020 m_agent - Meraki 0 Installed -
But in the Missing Apps, I also see :
Dec 29, 2020 MYPKG (all devices) No No
Reinstall
Dec 29, 2020 Meraki Systems Manager Agent Meraki (all devices) No No
Reinstall
Dec 30, 2020 Mozilla Firefox Mozilla (all devices) No No
Reinstall
So it's no so clear that the agent is installed as I thought first. Now I've clicked in the "Reinstall" buttons but if the apps eventually install, it does not appear to by dynamic.