I’ve done this but much more gin and much less elderflower liqueur (2oz & 0.5oz) and a botanical gin it’s quite tasty but I imagine if you put that much liqueur it would be WAY too much like SO much elderflower.
A botanical gin makes all the difference here. Also, using a gin bottle straight from freezer and cooled (fridge) tonic water and liqueur prevents ice from diluting too much. In fact, I can drink it without any ice when done this way.