It looks like the biggest domain count driver by far is image fetching for link posts, since rendering a website preview involves fetching both the preview image from the linked website and the favicon from Google (favicons account for all those t1.gstatic
calls). Disabling website previews and the associated image fetching code cut the domain list down to just Lemmy instances. Mastodon appears to proxy image requests through the instance, which prevents that high domain count. Lemmy has recently added that feature, but right now only instances running the very latest Lemmy code perform image proxying.
We're looking into adding enhanced privacy features to 2.0; we'll include one to disable fetching favicons and image thumbnails, since favicons especially are a known tracking/fingerprinting vector.