Is it possible to change it so that closed topics can also be edited, but only by their own authors?

The only way would be with a custom modification. There is no way to do what you're asking otherwise.

You could give members the moderator permission to lock/unlock content, but then they'd be able to lock/unlock anything not just their content.

59 minutes ago, teraßyte said:

You could give members the moderator permission to lock/unlock content, but then they'd be able to lock/unlock anything not just their content.

Nope, there’s a Can lock and unlock own content? per group setting, so the user would lock only their content. Same for HIDE and DELETE.

23 minutes ago, Adriano Faria said:

Nope, there’s a Can lock and unlock own content? per group setting, so the user would lock only their content. Same for HIDE and DELETE.

Right. I was thinking about that one but wrote about the global one instead. Ignore me. 😅

In any case, if they don't want to allow members to lock/unlock their content, the only solution remains a custom modification. 😋