I think both names sound really great as a native Chinese speaker and the characters you have chosen are very suitable to be used in names. Although I have to admit I have never seen the character 滢 before. It is not a common character but the definition is very meaningful (it can be used to describe something is as clean as the water), so I suggest you keep it! I don’t think anyone would have trouble pronouncing it since it is basically the character 莹 with the water radical.
As for the name 晨曦, it paints a wonderful imagery of the morning sun and it is definitely a great name (it’s the kind of names that main characters in romantic novels or dram series would have )
Personally, I think if you’re looking for something unique, 若滢 is a great one. It sounds young, elegant, and it has the element of surprise (being the 滢 character). If anyone asks you about the character you have a nice story to tell. That’s my thoughts on the names! Hope this will help.
Rebecca