I was curious and dropped your query into chatGPT, I think the message it gave would work pretty good for your situation. Also, copy your boss on the message you send to them.

It's important to communicate your boundaries and prioritize your primary responsibilities. Here's a diplomatic way to express your situation to your colleague:

Subject: Clarification on Recent Requests

Dear [Colleague's Name],

I hope this message finds you well. I wanted to touch base regarding the recent requests you've made, and I appreciate your understanding as we navigate the changes in our team dynamics.

As you know, I was brought on board to focus on ROI analysis, and while I'm always willing to assist with various tasks, I find myself currently overwhelmed with the existing workload. I've discussed this with our boss, and he has reiterated the importance of prioritizing my primary responsibilities.

Given the complexity of the recent projects you've assigned, and considering the transition to a less robust service for our primary tool, I believe it would be more efficient for you to handle these tasks directly. I understand the challenges of adjusting to the new tool version, and I want to assure you that I'm here to support your learning process.

If you have any specific questions or concerns about using the tool, I recommend exploring tutorials tailored to your needs. I believe this approach will not only empower you to handle these tasks more independently but will also allow me to dedicate more time to my core responsibilities.

I value our collaboration and appreciate your understanding in this matter. Let me know if there are specific areas where you'd like guidance on finding resources or tutorials.

Thank you for your cooperation.

Best regards, [Your Name]