This was very helpful, thank you @wilx!
In case someone finds it useful, this is how you apply wilx’s solution to a single post (under Appearence -> Customize -> Additional CSS). It effectively gives you a ‘new template’ (header image only) to play with in Twenty Twenty ??
———————-
.postid-123 .cover-header {
max-height: 40vh!important;
}
.postid-123 .cover-header-inner {
padding: 0 0 70vh 0
}