I thought that writing a post with more content inside could be more interesting for the reader

Yeah, it's indeed interesting, but when you participate in a contest/challenge you need to follow the given rules or else you will be skipped. Isn't this is how it works ?? You participate Battle Mage Secrets challenge where you need to create a content with the given ruleset theme, It's not like 1 person reading 100 other player's posts, it's 100 other players reading 100 or more posts. We all like splinterlands and we all like upvotes (who don't love money ?) but to get that you have to follow the given rules.

I'd also like to hear the opinion of Splinterlands' official curators, like @libertycrypto27 @mango-juice and @alokkumar121, because I put effort in writing my Splinterlands posts and, if my effort is worthless and I should change what I do, I'd like to know it.

Sure they can share what they think, But I believe your effort is not worthless, you just need to do it in the right way.

Untill 2 weeks ago people were allowed to write about what they like to support the game, as long as it was about Splinterlands, original and of some quality. If this has changed, that's ok, but I'd like to have an official confirmation, so I can change my schedule and start writing only about what we are all required in the challenges.

It's not like you are not allowed to write about what you like, you are free to write anything you want. But the challenge you are talking about it was social media challenge, earlier there was not specific theme there to follow but right now from past few week there is specific theme. so right now on both battle mage & social media challenge have theme to follow. But if you write a amazing splinterlands posts then SPL curators will support that even if you don't participate, it just needs to be informative and quality full.

I believe that the lack of different contents is not going to help Splinterlands onboarding new users

That wrong all, I think every Hive user know about Splinterlands game so there is not much to do with onboarding new users, we need to go outside this chain and bring new people.