> Merging all the related branches into master when doing a release ...
--- doesn't sound like Gitflow. In Gitflow, the topic branches have been merged into the develop branch first, as they were finished. In contrast, merging into master is a merge from a single branch. You branch from develop into a release branch. Then you merge the release branch into master.
What if you have multiple ongoing release (maintenance) branches whose changes should all be merged to master and develop? It quickly becomes messy (but to be fair so does most alternatives).
--- doesn't sound like Gitflow. In Gitflow, the topic branches have been merged into the develop branch first, as they were finished. In contrast, merging into master is a merge from a single branch. You branch from develop into a release branch. Then you merge the release branch into master.