@EtienneNadji Well, I feel stupid because this looks like an easy problem to solve.
I tried multiple times but with external content, I failed. I either corrupted the external content so that it's not of use any more or I corrupted my feed.
My current plan (for the far future) is to locate a library that may solve this issue for me. Which is not that easy because I do have a mix of handcurated strings/formats and externally generated strings/formats and feed format is some kind of weird to me.