How to amend a LTTextBoxHorizontal?

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I have a pdf page like the pic bellow.

I tried to extract the flowchart and I'm expecting to receive 8 LTTextBoxHorizontals whereas 3 LTTextBoxHorizontals in the middle combined together, like <LTTextBoxHorizontal(6) 166.615,195.469,509.322,527.856 'SURVEILLANCEb\nColonoscopya at 1 y\n\x17If advanced adenoma,...........5 ycc\n• See Principles of Survivorship (COL-H)\n'>. I've tried to amend the line_overlap and line_margin parameters, but the middle part splits in a strage way.
Consider that split should be based on 3 vertical lines on left of the text, how can I amend a LTTextBoxHorizontal and create a new one in order to get correct splits?
Attaching the pdf prob_page.pdf