The Idea behind the American SC was that their life long appointment would eliminate the need to be corrupt as they (theoretically) wouldn’t have the ‘pro quo’ part of ‘quid pro quo’ to corrupt them. In reality, that doesn’t seem to work calling into question the necessity of term limits and of course corruption checking.
Packing the court to a few hundred justices isn’t really necessary as it would just be more like the US Senate which does exist.
But I agree, they seem to have too much power as is.