Ignoring situations involving doxxing (which are more serious but also less common and easier to protect yourself from), the main goal of sustained harassment on here is to get you to delete your blog. You can probably keep most of your friends by adding them on discord or whatever, but the underlying logic in these cases is “you are a person who exists on the same part of the internet I use, and I want that to stop.”
I reject the idea that moderation will ever be a solution to this problem, at least not without creating new, worse problems. But tumblr can and should provide users better tools for defending themselves.
Let’s talk about tumblr’s private blog feature. You can password-protect a blog, so that users have to enter the password before being able to see and interact with your posts. This is barely a step above deleting your blog.
You need to manually share this password with everyone you want to have access to the blog, which does not scale to a following in the thousands. You need to hope that no one ever leaks the password to your harassers, in which case your only recourse is to change the password and manually reshare it, again. This is a Bad System.
What does a good system look like? Consider twitter’s private account feature. All your followers are preserved, but future followers have to ask to follow your blog. If you block a follower, they can’t just create a new account and dodge the block - they’re locked out. You keep your blog, you keep your following, you keep your friends, and no one outside your bubble can touch you.
There are still downsides here compared to not being harassed at all - people who don’t already follow you can’t see your posts - but it gives you a way to turn off the harassment spigot that can be easily reversed if/when your harassers get bored.
Combine this with a feature ideally for both private and public blogs (allowing only people you follow to reblog or reply) and I think the situation on this site improves a lot.