How is voting by mail any more secure or privacy preserving than voting digitally from your couch?

It preserves privacy by splitting the vote and the identification forms into two different envelopes. The vote (and only the vote) goes into a second envelope that is placed into the return envelope along with the identifying information. After the information is verified, the envelope with the vote is then added to a ballot box (and not opened right away). The whole process also happens in the open, so everyone interested in either the process or not trusting it to be properly done can stand by and watch. At least here postal votes are processed like this.

Sure. I'm still waiting for a description of how that preserves privacy? Until you seal the vote it's not privacy preserving. Literally the whole "privacy" aspect of the ballot is around making it impossible for anyone to observe your vote (& thus be able to punish you for not voting the "right" way). If you disagree, I'd like to bring North Carolina to your attention.

I don't know? I didn't make such a claim, but I'll speculate a little anyway. Vote by mail is accomplished by trusting the postal service which has a good track record of not losing or mangling my messages and, at least where I live, heavy legal deterrents against tampering. I imagine a digital solution could be as or more secure, but it may not be more trustworthy. Especially if the software running it is opaque or I don't understand the math involved.

Vote by mail is accomplished by accepting that a number of households/groups will cast a unanimous vote that would have been different votes if they happened in the enforced privacy of a voting booth.

That might be an understandable opinion if it is accepted as a conscious trade-off, but if you pretend that it will never happen you are just lying to yourself.

Most of the people I know claim to just follow the voting guide in The Stranger. I imagine they'd take a copy with them to the booth too.