Bobo, I am not convinced PS's are that complicated. My guess is that they are often like job applications for an attractive job. Even if they are read they are normally first read and scored by someone relatively junior and perhaps within the admissions rather than departmental team. You need to work out what boxes should be ticked and make sure you tick them. Interestingly, at least for medical schools, interviewers have often not even seen the PS and once you are through to interview scoring starts afresh.
DD ended up writing her PS under pressure of a deadline whilst still recovering from a serious and scary illness, which made it a bit more "all hands on deck" than it should have been. But still not hard. We downloaded all the guidance available from Universities and UCAS, and bought a book applying to medical school which had a brilliant chapter on how to write a PS. The first draft was a check list of things she had done etc which might provide evidence med schools were looking for, to discuss with the school, and then a case of her writing something following available guidance closely. She then got further comments from the school, mainly in the form of questions or suggestions for expansion, and then a case of editing to fit in with the word count. DD is dyslexic and does not write well, but our assumption was that it was content that mattered. And that the statement read very much as if she had written it. (One of her equally able friends had a very slick PS which read as if a consultant had been involved, and did not do as well in the process.)
Bristol medicine was an odd one as their initial filter relied very heavily on PS and reference scores (only 20% weight given to academics and no aptitude test). DD probably got through because she had the content and made sure that content was in the statement, not because of how well her statement was written.
My advice to someone applying from a school with limited RG experience, including French schools, would be to locate as much guidance as you can, make sure you tick all the boxes and follow suggested formats, and then get sensible readers (whether inside or outside school) to comment, but not rewrite. It must remain your statement.
And start early. It is the sort of thing that needs thought, review and editing.