In the next issue, I’d like to mix up articles and poems etc instead of the usual separate headings of articles, poems etc.
When I tried to re-order submissions, I can only do that within categories. I know I can reorder categories but that isn’t what I want to do.
I am using 3.3.0.17, OJS 3.3.0-4
Suggestions or advice welcome 
You mean sections? Create a new section and assign all articles to this one.
Sections are divisions within an issue.
Categories are across articles of different issues and may even be hierarchical (parent-, child category).
Yes this could be a solution. I have now created a new section of submission and put all submissions in that. Thanks,