Diagnose IPsec Tunnel Issues

My site-to-site IPsec tunnel was built following best-practice guides, yet the link keeps dropping for reasons I can’t see. The firewall on my side is an Opnsense appliance, and each time the tunnel fails the logs simply show that the connection drops intermittently without any clear error codes... (Budget: $10 - $30 USD, Jobs: Firewall, Linux, Network Administration, Network Engineering, Troubleshooting, VPN)