So there’s an existing feature request from a few years ago about toggling lines of text, not specific to tasks.
We had a big internal discussion about this a few times, and our conclusion always comes to the same thing – we’re dissuaded by the idea of adding collapsible elements within cards, as cards themselves should be the smallest unit of information. That’s also one of the reasons why you can collapse cards themselves already. That’s not to say we won’t ever add it, we always consider what our community wants.
Currently when we complete tasks within cards, we quickly move them to the bottom by selecting the text and using Option / Alt + ↓. If the card suddenly has 5+ tasks that are completed at the bottom of it, we select all of them and Move them into a new child card called “X Completed Tasks”. And then link to that child card at the end of the original card. Do these suggestions help your workflow?