I disagree that these two letters would be much help for English learners. E.g., though, through, thought, tough. Adding thorn and eth wouldn’t fix the bigger problem there.

Also curious what you mean by saying we don’t even use voiced and unvoiced dental fricatives consistently. As far as I’m aware, I’ve never heard an accent use an unvoiced one for the word “the”.

The 打 in 打火機 is more like hit in this context. Makes a bit more sense with the way older lighters required you to flick it. But 打 is also used in a ton of contexts haha.

The second character, 鵝, means goose. So 企鵝 is literally something like, “standing on tiptoes goose”. But no one would look at it that closely, to everyone it’s just a penguin.

“Threven” has a nice ring to it now that I think of it.