Landing page first! ok, but what if someone steals the idea?

Well, the answer lies within your question.

"what if someone steals my idea, creating a better product with better marketing?"

First of, ideas are worthless. Execution is key. You can broadcast your idea in the entire galaxy -- if you're confident you're going to execute it in a good way, you have no issue with people stealing it.
Second, "creating a better product with better marketing". If I'm going to steal your idea and make a better product with marketing, props to me. We both started with the same idea, but I executed better. Nothing wrong with that.

So, care less about people stealing your idea and care more about how to properly execute it.

Yesterday, I saw someone here post something about a small webapp idea. It is so small that I can't even call it a micro SaaS, let alone a SaaS. I was interested in expanding on that with its current owner but he said he's willing to keep as is. Guess what I'm doing now? I have stolen his idea and I'm building and expanding on it. I have so many things to add to it to make it appeal to more than just one category of people + make it bettter/enhanced/etc and a proper SaaS.

Am I liable for stealing it? No. He is definitely not the first one doing that thing nor am I the last one. But if I win the game against him is because 1. he did not want to improve and 2. I want to improve and EXPAND it. Props to me for having the willingless to build and improve and differentiate, not blames to me for stealing.