I have a requirement to split one big Team project in Azure DevOps to multiple team projects, as you know there are work items, iteration , repositories, test plans and CI/CD pipelines to move across. What is the best way of achieving this - apart from manually moving them across ?
Currently, there is no default way to move sources from one Team project to another. You can check OpsHub Visual Studio Online Migration Utility which can migrate source code, work items, etc.
Meanwhile, there are move work items (select some work items > Right click > Move) to another team project, move git repositories between team projects, move items in TFVC features that can help move sources.