If you have domain expertise or just domain interest, it’s easier to start a new subreddit than build a new platform. Because you offload a bunch of busywork like security and ddos prevention and account verification.
Of course UI comes along for free too, but content is king.
So content not UI is where to start if you’re building your own thing. That means focusing on a specific audience, not everyone.
And it means having exclusive domain knowledge and/or enthusiasm. And so that means attracting experts.
If that expert isn’t you, it probably won’t happen. Assume other experts already have sufficient online outlets for online expression.
Basically if you build it they won’t come. Build it anyway if you must. Don’t build it for numbers.
Good luck.