Hi Markus,
Usually, I try to use the rh-mariadb102 Collection but for the next document, I will try 103.
For PostgreSQL, I always use version-11 which now is at 11.6.
After installation, I used yum-plugin-protectbase to protect [pgdg11] in /etc/yum.repos.d/pgdg-redhat-all.repo. In that section I insert protect = 1 so the updates for PostgreSQL-11 will only come from [pgdg11].
For my documentations, I always prefer to use LTS as there are less suprises with them and also, I don’t have to update the documents so often.
On the other hand, I cannot recommend anything to you, because I consider that you know much more than me on these subjects.
As example, after playing around with Zammad, from now on, I will try to always use your Nginx installation as Nginx is the most used web engine, even more than Apache.
Michel-André