How to choose what to do first and what skills to learn, in a one-person kind of project?

Hey makers,

2020 coming to an end soon and I've been a little bit all over the place.
Started my journey in March with an idea for an app for workout and coming from a Python background worked with Flask.

After realizing the difference between an app and a webapp, I moved to Flutter to try to build something for both iOS and Android.

Having read a lot about MVPs and the sort of spreadsheet-based no-code apps I tried my next thing which was an app for plants-giveaway with a single FB post and Google Forms. It was quite a success (had almost 100 people show up for an event, and over 100 phone #s and sign-ups).

Given that sort of success, I'm now trying to build the MVP (have no $$ for a full-time developer...). People I trust recommended the MERN stack which I've been learning for the past month.

I understand that progress is not linear, but I feel like this has been quite all over the place, and if I decide to quit and look for a job, there would be no "hard skills" or a tech stack that I can say that I mastered.

Building on this year's experience, I'm determined to focus during 2021 on a much more specific path and am trying to decide how to go about it.
I have an idea for a SaaS app I'm pretty happy with and wondering how to start as a one-man project?

What tech stack? Maybe stick to google sheets and Figma before anything else?

Would very much appreciate your thoughts and advice!