To be clear you don't HAVE to. It is perfectly possible and acceptable to fly a 3DR Solo without any app connected. If you want add-on functionality such as is available in Solex or SidePilot then you will indeed have to shell out.
Or learn to use Mission Planner to interface with the bird. Or purchase an older phone or tablet which the 3DR app will still run on.
You have options, my friend. Options, choices, and decisions.
You would have to go straight to Open Solo 4 using Solex. The link is in my signature with instructions and guides. I don't generally suggest it for a brand new user that's never even gotten it working to begin with, but it does work.