Learning a design program to create your own plans is adding another layer to the almost insurmountable task of building your own tiny house from scratch as a first time builder. I want to caution you to engage a professional to coach your build. Builders spend about 3-5 years securing their skills as a builder. As far as the design, architects spend about 8-10 years polishing their design skills. For any one to think that either can be condensed successfully into a single build experience seems quite unreasonable. Hense, my suggestion to hire a coach for both processes.
My one suggestion regarding the plans is to “furnish” your plans with measured, scaled drawings of each piece of furniture and then analyze the traffic and flow patters in your design with your space being ” occupied” by both people and furnishings.
A common comment upon leaving my own 8 years in Architecture school was: “Don’t build your OWN house first!”