You could try flavour drops?
I haven’t tried RTD myself so can’t say how successful it would be, but have just bought some flavour drops to try in the powder. Worth a go maybe.
Or try cacao powder - I can’t think of anything that cacao powder wouldn’t make better
I tried to flavour vanilla RTD with the matcha flavour system powder, but it didn’t mix into the drink at all. I ended up with bits of matcha powder floating in my RTD vanilla. I like the taste of both though so it wasn’t an issue
I think the powder has gums and other ingredients that help everything dissolve / mix into the water, even whatever you add. If you add a spoon of cacao to a bottle of cold water and shake, it just goes lumpy. But if you add it to Huel Powder plus water it mixes really well.
I’m guessing the RTD doesn’t have these same ingredients as there’s no need.
Liquid flavour drops is the way to go I think. I use mint cake flavourings and more recently have bought my protein flavour drops. The hazelnut was surprisingly good! Edit: (I haven’t tried them in RTD tho)