If you are ok with a slightly less stable version (mainly an issue when piping fine details, swirls and such are no problem), and a more cream-ish color as opposed to the white meringue-based types, “German“ or custard-based buttercream could be your “low butter” alternative.
It’s made by combining butter and a pre-made and cooled (!) custard (or in a simpler version, starch-based pudding). Recipes and ratios vary and so does the type of custard or pudding. From experience, I can recommend the “whip butter first, then slowly add custard” method over the “whip butter into custard” method.
The flavor profile is less “buttery” and is influenced by the custard. But it complements cakes well and is pretty easy to make. The ratios are quite flexible, and I have in the past used as much as 2 cups of custard (starch-based) per cup of butter.