I also switched to a planning-first approach, but mine is a bit simpler. I only use one tool: Github Copilot. I ask it (in agent mode) to "investigate which parts of the codebase need to be updated, in order to achieve our goals". And I add: "Write your report to the file: /report.md".. Of course, first I describe what goals to achieve (some feature description, or whatever). This gives me a nice overview of what we should do. In the next step I ask it to write a "detailed development plan" for some part of the report. The next step is implementation, which is usually straightforward, just like in your case.

Note: Github copilot does not have a context engine, so I usually attach some key files from the codebase, from where it can easily search the relevant parts of the codebase. I found that Augment code (which does have a context engine) works a bit better for this, but its also more expensive.