Well your issue right now is you don’t know what you don’t know. How can you scope out a game when you have no intuition for how complex a certain mechanic or feature actually might be?
I think the best start for you would be to pick up a random course on Udemy and just get through it, see what the implementation of really basic features involves. You would then have a much better understanding of what further development, or similar pieces of work would require too. Linking a few below as examples, but I’m not affiliated with Udemy, not have I taken these courses.