Just downloaded another font (sense-5 for DB3350 mega-pack). Inside that is a .ttf, but it has another icon in Windows.
The font.xml config links Bold, Italic and Bold-Italic to the same one file. Installed it in /tpa/preset/system/fonts and it worked. Opera Mini works, all other java apps too.
So as you said my font had covered much more code points (made a comparison via FontLab Studio) that is why my device was struggling.