PF2e-TeX VS CARPS

If you're savvy and want to do longer, more complex stuff there's also https://github.com/serialhex/PF2e-TeX, but it's much more complicated and probably unnecessary for most use-cases.

Start by making a game design document. It doesn't have to be a serious one like you hear proffesional game designers talk about. Really just start with a thesis. Ask a question such as "Can a functional TTRPG using a D&D 5e core emulate MMORPG mechanics while maintaining the aesthetic of the Sword Art Online series". That's just an example, I really don't recommend 5e for a TTRPG that emulates a video game. You'll have to cut a lot out like u/octobod mentioned in their comment as D&D is class-based while the MMORPG in the SAO is skills based. It wouldn't be impossible, just a lot more work than other options. It definitely has its place in a toolbelt, 5e is not a great multi tool. Games like Cortex Prime, GURPS, FATE, or PbtA are much better tools to emulate a genre of your choice rather than being tied to a specific genre of the authors choice. Although they also have their own downsides. After you write a thesis, start on your design core (The Robbit Riddle case study that I sourced that article from is very helpful for new Game designers). You will return to this document constantly & edit it as the project is play-tested repeatedly. It is not set in stone, but try not to add so much that it becomes convoluted.

As for specific writing advice, I can offer very little as I am more of a technical writer & game designer who moonlights as a storyteller, but the work of Brian MacDonald might interest you. He's the guy that Pixar brought in to teach their writers the fundamentals of storytelling. His book Invisible Ink is good for writers/creatives. The Robit Riddle Case Study is also a good reference for designing a core concept. I also heavily recommend the book Rogue Genius Games - Writing With Style - An Editor's Advice for RPG Writers by Ray Vallese. I can think of a few other books & resources if you're interested, but these seemed like a good starting place.

There is a version on my GitHub, but the most up to date file is a local markdown file since it makes for quick formatting & easy Reddit posting. Sometime in the next few days I'll get around to importing that into a Google doc & I'll share that with you too.