For sure I am not the first one to have figured it out, but I couldn't find a thread either, so please bare with me sharing this idea, thanks.

PROBLEM: You want to create a non-primitive flowchart (or diagram/schematic/presentation) which includes symbols and math typeset from Mathematica (or TeX, MathML).

SOLUTION:

1. Create the flowchart with blank fields, in an application which supports exporting to the file format *.SVG, for example the popular free Dia if not Microsoft PowerPoint.
2. Install the function SVGImport on your Mathematica system. It requires you to also install Python on your computer and then to install the Python link.
3. Now you can import the SVG-file into your Mathematica session and edit the scalable vector graphics with the Mathematica Drawing Tools (Ctrl+D) and enter math typeset/formulas in the blank fields of your diagram, as you please.
4. The image information is completely stored in the *.nb-file (you can delete the SVG-file).

comment: While this procedure sounds all simple and logical, it's not trivial since several software versions are involved. It works on one of my older systems just fine (Win7, diaw.exe 0.97.2, python 2.7.18, Mathematica 11.3), but not so well on my Raspberry Pi 3B Stretch. Thanks to @Richard Hennigan for the wonderful function!