I'm curious to see these custom fonts! I'm saying "bitmap font" but I mean... I just want to see the font's import settings in Unity!
Basically, fonts that don't inherit from .ttf or .otf behave differently with the Ultra shader, and the way it's determined is in-shader is that they have a certain value (I'm forgetting which, specifically) that's negative instead of positive, s STM/the shader is checking for that. Perhaps you set up your font correctly (afaik, the way Unity does it is *incorrect* according to their own docs...) and this value is flipped so it's acting like the other font type. Or it could be some other metadata value causing it to be interpreted the wrong way, I don't know.
But yes, the STMMaskableGraphic component allows STM to work together with Unity's "Shadow" and "Outline" components (as well as LeTai Asset's "True Shadow" or any other custom Unity UI effect) so... that should be a good option!