DNS exfiltration is a technique that uses the DNS protocol to transfer data from a compromised network or device to an attacker-controlled server. DNS exfiltration can bypass firewall rules and security products that do not inspect DNS traffic. The characteristics of the suspicious DNS traffic in the question match the indicators of DNS exfiltration, such as:
DNS traffic while a tunneling session is active: This implies that the DNS protocol is being used to create a covert channel for data transfer.
The mean time between queries is less than one second: This implies that the DNS queries are being sent at a high frequency to maximize the amount of data transferred. The average query length exceeds 100 characters: This implies that the DNS queries are encoding large amounts of data in the subdomains or other fields of the DNS packets.

A rogue access point is a wireless access point that has been installed on a network without the authorization or knowledge of the network administrator. A rogue access point can pose a serious security risk, as it can allow unauthorized users to access the network, intercept network traffic, or launch attacks against the network or its devices. The first action that should be taken to protect the network while preserving evidence is to disconnect the rogue access point from the network.
This will prevent any further damage or compromise of the network by blocking the access point from communicating with other devices or users. Disconnecting the rogue access point will also preserve its state and configuration, which can be useful for forensic analysis and investigation.
Disconnecting the rogue access point can be done physically by unplugging it from the network port or wirelessly by disabling its radio frequency.

Security Orchestration, Automation, and Response (SOAR) can help the SOC analyst reduce the number of alarms by automating the process of removing duplicates and managing security alerts more efficiently. SOAR platforms enable security teams to define, prioritize, and standardize response procedures, which helps in reducing the workload and improving the overall efficiency of incident response by handling repetitive and low-level tasks automatically.