Former Java/Android Developer transitioning to Full stack - biggest challenges?

I have project ideas but are all web based. I just think there are more opportunities with web. Coming from Java/Android background, for those who could relate, how long did it take you to learn full stack and what were your biggest challenges? I have a project that I'd like to launch an MVP before the year ends. I'm planning to use MERN stack. Already did some research. So it will be more of learning by doing. Am I being crazy for setting this goal? 😅 Any advice would be greatly appreciated. 🙏