I was just reading some basics…
Why do you think SRB2 came into the picture when SRB1 itself is capable of transfering NAS messages?
Is it just to define a low priority SRB for NAS messages?
According to @Jaeku_Ryu (Sharetechnote):
“SRB1 is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the establishment of SRB2, all using DCCH logical channel”
So NAS messages are transmitted over SRB1 till SRB2 is established in 5G.
But the same can be achieved by SRB1, why introduce another SRB?