I already tried something small, which turned out to be a wild goose chase, because I had no clue how micro.blog works under the hood (templating, Hugo, etc.). I’d rather not be in that situation again. Hence my question: “How do I get started from nothing.”
Luckily, there was a solution, but not very satisfactory, because it involves copying HTML source code from the /archive page, and adding categories manually. Since I can’t expect any of you to solve my particular problems, the only thing I can think of is to learn templating with Hugo and apply that to micro.blog.