Hello SunriseSunset and welcome
Hope you decide to join - it's a great place and you learn so much, as well as getting a lot of support and empathy.
Can't answer the question about copyrighting, though I look at it that anyone can steal an idea, but no-one can write it the way you wrote it, and if you've written it you're ahead of the game anyway, if you see what I mean!
Like you, I also waded through every page of the Getting Published Forum, which is v. addictive!
As to your word count, I'd say yes, you need considerably longer than 21,000 words for a thriller. Though the crime writers here will have a better idea. The 'average' word counts for adult fiction begin at about 80,000 and go up to 150,000 for saga type work. Naomi M might be able to say whether your work would count as a long short story? I think even novellas tend to be about 50,000 words - but again, see what others say, as I'm writing Women's Fiction.
Anyway, well done for writing as much as you have, and hope you join us.
Susiex