How can I import/rewrite settings/content types/blocks/etc from live with the newly developed features on dev? I have a site that's already up and running. On my local dev environment I occasionally develop new features for that site.

I have certain configs(blocks, settings, etc) that I need to add to the live site. However, I can't seem to figure out a way to import them without deleting the current blocks/settings/etc that already exist on the live site.

Is there some way that I could import them on the live site without deleting them beforehand?
IMHO, using git for that works quite well.

You have a production branch and a development branch (and possibly many feature branches). Locally, you are on the develop branch, you make your changes there, export and commit your changes there

Before you do a deployment, you switch to production branch, do a config export ,sync it down, commit it, merge develop into production, do a local config import to make sure that things are consistent and then deploy.

A useful presentation for this is https://de.slideshare.net/nuvoleweb/advanced-configuration-management-with-config-split-et-al, for example slide 39, although that focuses more on working with multiple developers, but the process with production is similar.