I think an electoral threshold of 4% may be needed.

I have absolutely no problem with such a threshold.

I also think it critical that the supreme court of the US isn’t 7 judges.

Okay so here's a really controversial take. I think the problem with the SCOTUS actually stems from there being too many rights enumerated in the American constitution. I should note that I'm not a legal scholar, but I've read a lot of opinions from non-American lawyers who have explained this viewpoint, and it makes sense to me.

Where I live in Australia, our constitution is largely uncontroversial. It doesn't say what rights people do and do not have, but really just lays out the basic functioning of our democratic institutions, like how elections work, how Government works, how the Commonwealth interacts with the States, etc. Rights are left to Parliament to implement. This has the interesting difference from America in that it means that our High Court decisions are largely far less political than SCOTUS's. Because the High Court of Australia doesn't get to make the inherently political ruling of deciding how to interpret individuals' rights as laid out in the constitution. By putting the right to bear arms in the constitution, SCOTUS is inherently given the power to decide what should be a legislative matter of how much people are allowed to own guns. It's what lead to the morally-good but legally-nonsense decision that lead to Americans having the right to abortion*, which itself stopped the legislature from ever feeling like it needed to do its job in relation to abortion protections, which is in turn what made the disastrous outcome of Dobbs possible.

This is, obviously, something so deeply ingrained that it would be basically impossible to change. Americans view their constitution almost like a religious text. Even though some of the founding fathers supposedly thought a constitution is something that should be basically rewritten from scratch every few decades, Americans view it as written in stone and as something that must not be changed except perhaps to enumerate more explicit rights. But fundamentally, a less politicised constitution would lead to a less politicised judicial system, which would allow each branch of government to do its part without encroaching on the others like they currently do.

I'm with you on increasing the size of the legislature though. 2 senators per state is far too few (and makes it impossible to reasonably add in a proportional system on a per-state basis). I have much the same feeling about my country. I'd like to see our Parliament almost doubled in size, especially if we were to move to a more proportional system (we currently have a proportional Senate, but use IRV for our House of Representatives).

* legally nonsense because if you look at how SCOTUS justified it in Roe, it just doesn't make sense, legally. Somehow the right to an abortion is derived from...a right to privacy? That doesn't make sense. And it makes even less sense when you consider that the right to privacy itself is somehow derived from the right to due process and equal rights under the law.