First of all, I would maybe try to reach people through commenting, even though it could be considered as spam and flagged. Sending wallet transactions seems more like spam and will less likely be read.

But on the other hand, it can't be flagged as spam, like the comments would. I think there are even bots automated to look for "spam comments", so I can see why you're approaching people through transactions like this.

But then to the question.

I would focus to the top posts. If I was to reach out to even top 10 level, I would leave only top 3 with high rewards, as in the example. It's possible not to even attract that many really good posts, so I would keep the top 1 as the clearly biggest one. It could actually a few good authors to participate.

Then on the other hand, giving 1 or 2 Steem rewards for all people who participate according to the rules would be a good idea. If it was left only as "all people who participate", there would be a risk for a bunch of participants who don't actually follow the rules.

I've seen also somewhat popular people have contests and getting very little participants and mostly low quality. This is one of the main reasons why I would prefer focusing the rewards to the top, as you don't always get even two good entries.