looking for an feature similar to the frac function.

[tapy]

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.

[mekkablue]

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

Features, part 4: positional alternates | Glyphs