Might be worth looking into Portfolio Planning/Strategic Planning instead, as the Workspace and the roadmaps that come with it are more user friendly.
As for the CIO Roadmap, you can try and have a play around within the Timeline Visualisation record (roadmap_page) - it is a High Risk file to configure as pointed out by the system.
You would need to replace the panel table to pm_project_task and in the condition to reference the Milestone = TRUE, however after looking into this only briefly it might be a bit more complicated than that.
I'd advise you to analyse the OOTB record first and to do that properly, display all the fields which drive the behaviour of the roadmap by turning off the OnLoad Client Script "Hide Fields" for roadmap_page table. Then try various scenarios to see if it's possible to configure it as you need for your use case. I had an attempt (2nd screenshot) which hasn't worked, but perhaps there is a combo that could.
Hope this helps - good luck 🙂
