they'reHISchickens :
Rehome BOTH and get two compatible hens of the same breed?
I have more than one flock. I have one that is 13 Dels and has two roosters, BUT there are also Frenchy a bantam friz and Buckwheat her daughter in the mix and they do fine.
Now my other flock is an assortment of Orps and two RIR's and some EE's and a Lav Amer. That group has ,,,, are you ready,,,,,,,6 roosters. Only minor problems there. Looking for homes for two of the roosters. Not bad mind you. The third flock is 11 BR and 2 SLW of which there is one rooster. The Br's and the SLW's are finally eating together.
Roosters are good for flocks as they can keep the peace between the girls.
What I recommend the OP do is get rid of the RIR hen and bring in two either BO's, Dels , or BR's hens or of course Sussex. I would recommend the new girls be the same age but younger than the Sussex but not chicks. Of the three I mentioned the BO's are the friendliest and that's saying something since I prefer Dels.
Temperament is an inbred thing with animals and something to be bred out IMO. More times than not I read about a RIR rooster who is not nice. Unfortunately they are also big and good for the table.
I wish you well,
Rancher