I'll be glad if my ideas prove useful :).
When I mentioned www.figma.com, I didn't mean you should focus on designing for mobile applications. Since you're not just writing code but also designing the game interface, it's more convenient to visualize it (create a screen layout) in www.figma.com and then write code based on the layout, adding buttons, text fields, and so on...
However, if you're interested in writing native code for mobile applications without using Unity, you can do so using the programming language "Objective-C" along with SpriteKit. I think it won't be difficult for you if you already know "C#." I prefer using "Swift" as the programming language, I like its syntax, which seems a bit simpler than Objective-C.
I'm sure it's worth uploading non-commercial code to GitHub; it enhances your GitHub profile, besides being convenient and free, it also helps to find like-minded people.