How do I organize the development of my game?

I’m a beginner in game development, but I’ve chosen Godot for my first project due to its intuitive workflow. My goal is to develop a 2.5D Top-Down Bullet Hell MMORPG. While I recognize this is an ambitious, multi-year undertaking, I’m committed to the process.

I plan to use some free 3D assets from the Unity Store and port them into Godot. I’ve already brainstormed the core attributes for items and player classes, but I’m looking for guidance on the development pipeline.
- Specifically: Should I build the item and class systems first?
- Additionally, can I develop the game as a single-player offline experience initially and migrate to a multiplayer backend later?
- How do I organize myself in a way that I know what to develop first? For example, Movement? Classes/Items? Assets/Sprites? The world?
- Should I be using software like Trello to organize everything?

I just don't know where to start, I'm afraid if I start doing something, I'll eventually have to change or completely delete it later on because of some compatibility issue or something that I still don't recognize or understand.

I understand the best way to learn is by doing and creating, but even if I have a clear idea of what I want, should I really be making other types of games that have nothing to do with what I want to create, even if it's for a learning experience?

I have found some really good videos in YT that explain a lot about game development and I did found the part of creating the world fairly straight forward, I think my main difficulty will be to actually develop the code for the stuff I want (C#), so if you guys could point me towards some good guides regarding this I'd be much appreciated, I do have some coding knowledge, nothing game related though, mostly web and software development.

Thanks!
Sorry for the long read.