looking for an feature similar to the frac function.

I made the fraction glyph into an empty glyph and made do-ko.numr and jea-ko.dnom glyphs and applied them to the numr, dnom, and frac features.

However, this method is difficult to use.

Is there any other better way?

If I make this into an alternate glyph I need to make a conjunction with over 10,000 characters.

Perhaps init and fina in combination with calt? There’s a tutorial about it: