Hi there, this is a tough one. I'm not sure about how to deliver something with that exact scenario, it may be possible with a couple calculated fields.
But to take a step back, what is the team wanting to know specifically/how are you defining velocity? Based on what you're saying, are you more wanting to get insight on how many stories/tasks the team is completing within a certain timeframe, or the speed at which they're closing on tasks? If it's the former, you could do a task or issue report (whatever they work in most) of completed tasks/issues MoM or QoQ pulling in anyone on that team.
If it's the latter, you could do a report showing avg time of when a task or issue was submitted to when it was completed on avg. If you have a way of categorizing your work in a custom form, you could see how many days per task/issue type on average from entry to completion. I do this with our finance request types, example below. If you're interested I can share more details on this report.
If this helped you, please mark correct to help others : )