Legacies reality change episode is very confusing

All the question you asked don't have a word to word answer in the episode itself but we can analyze from what we know already.

Has anyone wondered why Lizzie's last wish didn't have that big consequences on reality?

Because she was jinn who need to complete her 500 years of servitude to be free but in her 499th year she meets the true monster and then ends up in Malivore. So by the final wish, Ablah never meets that monster so she finished her servitude and got free. So she left with less than a year to fulfill anyone's wish but as we know from her cleverness she might have done nothing impactful for history. Previous wishes of Lizzie messed with the world more due to it being so selfish and related to Hope which was part of the rich history, Hope is from infamous Original family and only tri-breed.

Having Jinni (not erased from the timeline) should have had deeper effects than to just bringing back all to normal except Liz with a confused mind. Or did she remember her experience?

We can assume that in the remaining 1 year she tricked other also as she tricked Lizzie. She never fulfills any of Lizzie wish in a good way and only made her trick her freedom.

Also if going from one reality to other, erases that reality's memory, then how did Liz know all of it until the last change?

Because in previous wishes she never changed Ablah's history and Ablah need to stick around her but in last wish everything changed, Ablah never have to come to Salvatore school as she must be free already.

What if she would have accepted one of the reality? would she be then remembering all of it or not?

We don't know.

Has anyone wondered why Lizzie's last wish didn't have that big consequences on reality?

As @ankit-sharma indicated, wishing "Jinni" never met the monster that sent her to Malivore means that she never enters Lizzie's timeline and thus, the timeline is restored to the original state.

Having Jinni (not erased from the timeline) should have had deeper effects than to just bringing back all to normal except Liz with a confused mind. Or did she remember her experience? Also if going from one reality to other, erases that reality's memory, then how did Liz know all of it until the last change?

About 47 minutes into the episode, when Lizzie is formulating her final wish, Jinni explains that the wish has repercussions. Everyone that has ever used their final wish to return everything back the way it was, ends up going insane. Jinni goes into more detail stating that they all (subconsciously) remember the alternate timelines and it slowly drives them wacko.

What if she would have accepted one of the reality? would she be then remembering all of it or not?

Yes. Jinni is very specific that the (person wishing for the) alternate timelines maintain memories of the events. Not in the same way that we would remember what happened the day before, but the memories are there. Like a seed or a thought, similar to a Déjà vu I would suppose.