Is there a forum / platform / website where people exchange thoughts on pros & cons of different startup business ideas (ie. SaaS products)?
Not that I know of. But if there is one what will prevent others from stealing your idea? NDAs won’t help
Never a guarantee. If it's a good/profitable idea than someone already thought of it or will soon. Either they aren't taking it seriously or they are taking it more seriously than you. Just work hard on bringing it to market and increasing brand visbility. Work on a small niche of customers so you become the "go-to" tool, version, or gadget, etc...
If you want to learn how to polish your “elevator pitch” just use twitter to discuss it.
Yes you release a prototype quickly and obtain market feedback
You will get feedback on prototype, not the idea
Feedback on ideas alone is garbage. Customer behavior is the real truth. So get market feedback as soon as possible.