Welcome to the design side of the fun!
I don't in any way want to slow down your enthusiasm - but it could be helpful to think about what your goal is with this project for just a moment in case it will adjust your focus. Are you looking to learn how to create plans that can be shared with others? Are you looking to be able to build any plane you want for yourself from scratch? Are you wanting to try out some techniques or design ideas and see if they work? Do you just want the joy of flying a plane you designed and build? Some combination of the above?
Depending on what your goals are, you could take an existing set of DTFB plans that are close in size - like the FT Storch - and set about modifying and pushing edges around to make a plane that looks like the Decathalon. That can work out great as you're adjusting from a proven airframe design.
As an alternative, you mention you have a set of balsa plans, so you could look at replicating the ribs and formers structures with DTFB and skin it with foam board too. Going down this path, you will probably be able to eliminate a good number of those ribs and formers from the plans due to the strength of the foam board skins. In this path you might not need to even create a new set of plans, just build away from what's currently there.
And for a "blank page" approach to designing,
@nerdnic did a great video series on getting started in DTFB design techniques over here
you can use photographs of the existing plans to get the outlines transferred into the computer and then start drafting along. There are some great CAD templates and tips for designing DTFB with the free DoubleCAD XT program over here too
https://forum.flitetest.com/index.php?threads/ft-style-cad-templates.25514/
And there are a whole bunch of folks designing crazy aircraft over in this thread that are also sharing tips on how to design and sharing how they are overcoming the challenges too
https://forum.flitetest.com/index.php?threads/2019-fantasy-aircraft-design-build-challenge.54677/