Agile Release Planning
In agile release planning, the unbound constraint is usually scope. So, an agile project generally has a fixed delivery cadence, fixed time-boxes, or a fixed number of iterations, but in general when to expect a piece of functionality to be delivered is only a forecast calculated from historical lead times rather than a fixed date.
There are certainly methods of delivering fixed-scope via agile, but they generally require treating schedule as the flexible constraint. This is probably the opposite of what you want.
One-Page Project Planning
If you're being asked to provide a one-page project plan, it's certainly possible to do that, although whether it's useful is debatable. I've bought (and personally disliked) the One-Page Project Manager agile template, which may meet your management's goal.
For myself, I found the template to be too "visually busy", and felt that it didn't really fit my conceptions of core Scrum and Kanban planning practices. In particular, the template and guidance seem very opinionated about release planning, estimation, prioritization, and the whole concept of done/not-done in ways that run counter to my own Scrum implementations. It also calculates burn-downs based on idealized hours and percentages, which is something I inherently dislike.
Despite my personal opinions about the template's suitability for my projects, it certainly seems like the concept (if not necessarily this specific implementation) would suite your needs well. You may even find that the template works well for you, even if it didn't work for me. It's definitely one of those "your mileage may vary" types of things.