Does it have to be post-coding, pre-merge code review?

The difference lies in size of the diff and the manner of introducing changes. Other developers have your changes instantly. This is core of the discipline called „continuous integration”.

I’ve been in multiple teams doing mandatory, pre-merge, double code reviews and getting crappy results. The longer someone kept their precious feature branch isolated, the worse.

And keeping branches „up to date” only solves a small part of the problem with long lived branches.