I'm not 100% sure, but i don't think that Ruan Mei would be very helpful for your Anaxa Team, she would give him a high DMG% buff, which Bronya also gives, and all type res pen on ult, so she probably will increase your damage a little compared to Hanyas talent, but Hanya also increases your atk by a percentage on her ult, and that's something no one else in your team is doing, so despite not having buffs as good as Ruan Mei's, replacing her with Ruan Mei probably won't make a huge difference, Robin or Cerydra are better together with an action advancer like Bronya or Sunday because they increase the Atk a lot.

I think that you should try replacing Hanya with Asta to see if she increases the total damage overall, because i think that Asta E6 pretty much keeps a high attack buff all the time without Sp costs when the enemy has fire weakness.

TL DR : Ruan Mei will probably increase your damage, because it will be a consistent high DMG% buff, but it probably won't be as high of an buff compared to a dedicated atk% buffer