There is truetype2gfx https://blog.adafruit.com/2021/11/04/truetype2gfx-converting-fonts-from-truetype-to-the-adafruit-gfx-library-for-arduino/
Not really familiar with any online tools for removing unneeded characters, but I have seen some discussions indicating they exist. Editing a GFX font file manually is not something that is easily done.
Which particular font are you using? The default font is based on the code page 437 font, not sure if that is available as a truetype font.