So the issue is the certification configuration for Elastic Agent and Fleet, which Synthetic Monitoring uses. Until you fix the agent certificate issue the monitors will fail.
I did a quick scan of your configuration against the SSL/TLS guide and I can't see any obvious misconfiguration.
I assume you are using a self-managed fleet server? I would recommend checking out the configuring TLS guide and also see if you need to specify the certs in Kibana as outline in this similar issue.
Alternatively you could see if regenerating the certifications is an option.
Can you try either of those and let us know if that resolves the issue?