I’m not sure if that’s possible either, unless you bake the effect into the font itself. If they were always going to be the same colour this wouldn’t be a problem, but I remember your screenshots showing the inner colour being different.
I think the best you can do here is to just batch styles and render all text of each style to a render target, post process the effect, then layer them all together.
That said, with the graphics you’ve shown thus far (quite pixelated), and using down-scaled render targets, modern computers should be able to handle a lot without significant performance losses. If it does, you might just have to rethink how many text effects you want, or limit the colours for some of the effects and bake those visuals into sprites so you can avoid post processing?