The ultimate goal of an anti-phishing campaign is to reduce the risk and impact of phishing attacks on the organization. Therefore, the most relevant and reliable indicator of the effectiveness of an anti-phishing campaign is the decreased number of incidents that have occurred as a result of phishing. This metric shows how well the employees have learned to recognize and report phishing emails, and how well the security controls have prevented or mitigated the damage caused by phishing.
References = Five Ways to Achieve a Successful Anti-Phishing Campaign; Don't click: towards an effective anti-phishing training. A comparative literature review; CISA, NSA, FBI, MS-ISAC Publish Guide on Preventing Phishing Intrusions

An increase in false negatives would be of greatest concern when reviewing the performance of an organization's IDSs, because it means that the IDSs are failing to detect and alert on actual attacks that are occurring on the network. False negatives can lead to serious security breaches, data loss, reputational damage, and legal liabilities for the organization. False positives, on the other hand, are alerts that are triggered by benign or normal activities that are mistaken for attacks. False positives can cause annoyance, inefficiency, and desensitization, but they do not pose a direct threat to the security of the network. Therefore, a decrease in false positives would be desirable, and an increase in false positives would be less concerning than an increase in false negatives.
References = CISM Review Manual, 16th Edition, page 2231; Intrusion Detection Systems | NIST

The main reason to involve stakeholders from various business units is to gain their acceptance and buy-in for the information security policy. The CISM Review Manual underlines that policies are more effective and enforceable when stakeholders understand and agree with them, which also aids in smooth policy implementation across the organization.
Reference:ISACA CISM Review Manual, 16th Edition, Page 45, "Stakeholder Involvement in Policy Development".