Comments do the same thing

Maybe at a very high level, but comments have the very obvious advantage that they provide something that moderators can block. Lemmy does have open voting logs, but I highly doubt any decent moderator would feel comfortable blocking people based purely on how they vote, and they'd only actually look if there was an obvious problem (e.g. maybe they need to consider blocking an entire instance).

directly incentivizing only interaction via people with the time to type up a comment

This only applies to negative interactions, you would always be able to upvote a post.

I think there's an argument for hiding the voting buttons inside of the comment thread so users can't just drive-by vote without actually looking at the comments, much less the linked content, but that's not what I'm arguing for.

You cannot police opinion quality

You're absolutely right, but you can increase the effort needed to downvote something. A downvote tends to have more weight than an upvote, so it should require more effort as well (e.g. a post with 8 upvotes and 0 downvotes would probably be ranked higher than one with 20 upvotes and 12 downvotes).

it sounds like you want to build a personal group chat, not a social media site

No, I definitely want a social media site, I just want everything distributed, including moderation.

Basically, I want something like BitTorrent, but for social media instead of files. That way there's no central authority for pretty much anything, so moderation pretty much has to be opt-in (otherwise you'd pick a different client with different moderation). Ideally, you'd select a moderation team that would filter out bad stuff like CSAM, but not filter out high quality content that you simply disagree with. So you'd pick a diverse set of content moderators to trust, and content would only get filtered out if a certain number of them flagged it. You could use the tools to create an echo chamber for yourself, or you can use it to expose yourself to diverse, high quality content that may challenge your beliefs (my personal preference).

That said, things tend to work differently in practice. At the very least, I'm not going to release it until I have a way for users to review the quality of the moderators they pick.