Xuanzhang or raikou

*obligatory "roll whoever you like" advice

Looks like you lack casters as what you have mentioned, so rolling for Xuanzang is a fine addition to the team. Xuanzang is a ST NP spammer like Medea but much consistent (I think) and can offer party support. Raikou is generally used for fast farming and while Lancelot, Fran and Tamacat has quick NP, they will still perform similarly to Raikou.

Personally, I will still for Raikou because being a Berserker she has advantage over other classes while Xuanzang is assassin killer like Medea. And again, Raikou is limited and Xuanzang can spook you once Camelot is released.

Something you should also know is that Xuanzang will be added to the gatcha permanently when the next story chapter is released(in just about a month)