The only thing ECHR cares about is one piece of "legislation", which is not a law, but a declaration (Declaration of Human Rights), so that you have some sort of internationally recognised body to go to whenever you feel that your local judicial system has done you injustice. That is all it does. That is all it is meant to do. That is the sole reason of its existence. It is not a legislative body at all.

> ECHR can simply invalidate national law.

It can't. You're either making things up or severely misunderstanding the court. It can say "this law doesn't align with the Declaration" and that's it. The law still exists. ECHR relies on signatories being willing to make the necessary changes themselves. Some are and get right on it, some aren't. The election law in my country has lost 5 cases in the ECHR and not a single one of the verdicts are fixed as of now, the oldest of which dates back to 2009. This is horrible, I want to see them fixed, but ECHR can't force us to fix it and we as in the country face 0 consequences for not addressing any of them (as of yet).

There is a separate court called European Court of Justice which is the equivalent of the US supreme court and is tasked with interpreting EU-wide laws and making sure national laws are aligned as much as possible. That is a legislative body with an enforcement mechanism. ECHR is not, you don't know what you're talking about.