Individually, both are good choices, but not next to each other. If Ashish is Rama, I don't want Mrunal as Sita, and vice versa. Hence my other suggestions. Those 2 don't look remotely close in age.
Tanvi Bhatia, who was Arpita in Gunaho ka Devata, has that softness in her that would nicely project a Sita. Conversely, if Mrunal is Sita, then Shravan Reddy has the gentle tone that people expect from a Rama. Only that you'd need younger looking guys to play LBS.
Another point - if this is a Sita centric Ramayan, I'd expect that UMS would be more than merely peripheral characters - at least preceding the 2 vanvas's
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