Fontconfig: how to create for Mono font synthetic Bold/Italic fallbacks (anybody)

[amerlyq]

We have only partial support for Mono styles, e.g:

┃ U+2190 – U+21FF Arrows                            ┃  ■  │  ■  │  ■  │  ■  ┃  ■  │  □  │  □  │  □  ┃  □  │     ┃

Fallback to double-width arrows is inappropriate for at least half of terminal usecases.
And fallback to any of Source Code Pro, DejaVu Sans Mono or Symbola is unsightly.
Many fonts are visually incompatible with PragmataPro and difference in Italic/Bold for symbols is simply too much.

But I can't even start wrapping my head around how to supplement synthetic Bold/Italic styles to font:

• /etc/fonts/conf.avail/90-synthetic.conf
• https://wiki.archlinux.org/index.php/Font_configuration#Create_bold_and_italic_styles_for_incomplete_fonts

Does anybody have any success story in taming fontconfig or any ideas how to avoid manual editing of font cache entries?