I haven't really seen as much congestion in these lately. However, if you are adding these over to the new mission battlefields, it'd be best if you made each non-Ark Angel fight drop only one type of Rem's chapter. So like....
Stellar Fulcrum only drops Chapter 6.
Celestial Nexus only drops Chapter 7.
Tenzen only drops Chapter 8.
Ouryu only drops Chapter 9.
Some still unimplemented BCNM drops Chapter 10.
Alternative, implement 2 new AMAN Vouchers. When turned in to the sparks trading NPCs, these vouchers entitles you to either 1 or 2 copies of any Rem's Tale chapter you want. Make that drop from the new BCNMs.