First time developing a program and I need some on my next steps.

So in summary,

This is a personal project I'm doing, and, in short, this is the first time I develop something I intend to distribute to others and it's safe to say I have no idea what I'm doing right now.

All I have so far is a very soon to be fully working back-end and I'd like to plan ahead while I finish it off and get some suggestions.

I have a to-do list of some sorts but the specifics and the methods I'm fully blind on.

• I need first to make a front end GUI for my back end, and I was hoping someone could recommend an easy to use, open source tool I can use (I never needed to make one before). The thing is my back end has both C++ and Python code, so I don't know if this complicates things.
• I'm planning an alpha testing sort of thing with friends so I can get some feedback and hopefully discover as many potential issues and bugs. I have a list of things to watch out for during that as well, but
  • I have no idea how to distribute the program. I'd also prefer to do that in a way they can't just access the back end.
  • And I'd also like a way such that, during that stage, only the people I gave the software access to can use it (i.e. if they give to a friend, said friend can't use it).
• Passed "alpha-stage", when I start distributing it on a more open form, I have WAAAYYY more concerns.
  • First I'm assuming that distributing it to a larger crowd requires a different platform or method
  • How do I collect feedback from a larger user-base (assuming I get one obviously)
  • The software tracks statistics the users want to track, I'd prefer to make it accessible both locally and online (I was thinking Firebase, because it's the only one I've ever used). But I have so many concerns such as
    • privacy/security – I don't want my software to end up being some backdoor for all my users' PCs
    • cost – my budget is one large O

This is my to-do list, so if there is anything else vital I'm missing that needs to be 'to-done', please let me know.

So really any advice on ANY of this would be really appreciated. Thanks in advance. : )