I would suggest taking a photo under different lighting conditions, uv is a thought but being graphite based, IR would seem best to highlight the writing, but I could not say which would be better, same angle from beside the camera or from the side slightly above the plane of the bespoke postcard.

Oh if I'm not mistaken, postage date suggests 111 years old.

Edit about 30 min later.

A thought, might be (iron gall) ink and not pencil. UV might be useful. [1]

[1] https://aiccm.org.au/network-news/summary-ultra-violet-fluor...

I did some fiddling in GIMP, including edge detection filters. Not much progress.

The uneven illumination might call for working with regions separately. I didn't see an option to even it out. Bright areas seem blocked/saturated (in brightness, not color)

Unlike film, digital sensors seem better at letting you bring up detail in shadows and worse with highlights, which seem blocked.

"I don't use algorithms" (separately) unless they're incorporated in GIMP. More a photographer than anything else. Few words were readable.

Saturation curve in photo editing software possibly broken down by channels.