@adrs On a side note:
Dynamic.Symmetry.zip (12.1 MB)
Maybe these will come in handy, assuming for the moment that you don’t already created these yourself. 50 or so dynamic symmetry based, multi ratios PNGs. The lines are black (easily changed to any colour) and the backgrounds are fully transparent.
I use these at times in Krita. The only thing you need to do is scale them up/down.
This does not solve your current darktable request but it might save you some time and/or include ratios that you do not yet have.