I originally thought this, I found it better when blended and chuck in a banana. Add in frozen fruit and its even better. I now use oat milk and that solves it.
The new strawberries and cream I never had any issues at all.
Using a blender (although this makes little to no difference from my experience), leaving to soak and using milks instead of water are the only things you can do to make it smoother I’m afraid.
The ready to drink is much smoother, but it’s a different product with different ingredients.
A blender would be the best solution, but if you can’t or don’t want to buy one you could try a metal spiral instead. Adding ice cubes, cooling and leaving some time to soak also helps. And it is important to first add water and then the powder.