> asking online usually leads to unhelpful or outdated advice.

Solution:

> This platform works like Stack Overflow, but for taxes — anyone can ask, answer, or upvote

Your platform is basically still "asking online".

What will ensure that the answers here will be helpful and not outdated, unlike these other places online?

If it's the voting, this is available in many other, already popular, platforms.

The above is based only on the OP, I didn't go past the landing page. Why is sign up necessary for seeing the discussions?

I can see Q/A on SO without logging in. On your site I can only see the login page.