Has anyone thought about building something like Twitter blacklists for Reddit so a group of mutually trusting subreddits can share their list of who they ban (for reasons not unique to their own rules) to a list then all ban that person/account proactively?

Doesn't matter because reddit accounts don't hold the same weight as twitter accounts. Making a new reddit account is trivial and you lose absolutely nothing.

is that not true of Twitter as well?

On Twitter you follow people, on a site like Reddit you subscribe to subreddits.

Switching between accounts on Twitter means your follows/followers are lost, switching accounts on Reddit doesn't lose you anything (unless you're subscrived to private subreddits or are a moderator for a subreddit)

Twitter sometimes requires a verified email and phone number, which I assume they globally ban when they can. Not sure how effectively they are, but they do at least something make throwaways harder than Reddit does.

Aren't echo chambers a more real problem than the always fuzzy "hate"?

The mods only have themselves to blame. They create insanely broad rules allowing them to ban anything and then limit mod positions to concentrate their power.

The role of mods is to delete off topic submissions and remove illegal content. Nothing more.

The role of mods is to establish and maintain the community they wish to have in the space. That takes a lot more than deleting off-topic submissions and removing illegal content.

And deleting off topic submissions and removing illegal content is what they do the most. Just because you read that some seemingly related post got removed, does not mean everything the remove are those.

I admit, I don't understand the tendency of people like this to insist that their hyper-libertarian ruleset must be so. That for a community to have its own set of rules that they enforce is inherently wrong somehow.