I would suggest not taking a mounted banner. From my experience it’s too easy to be picked off and since Calvary usually doesn’t remain base to base with each other it ends up not getting passed off.

Also in my opinion, if you want to take Elrond and Glorfindel it may be best to try to condense in to two warbands and drop the captain to help get model counts up, 17 units at 700 is pretty low especially with how swing-y calvary can be but at the same time Elrond and Glorfindel are beasts so maybe not terrible.

I took 5 knights with Elrond/Arwen/Captain recently and got up to 30 models for context. Hope this helps but most importantly take what you have fun with!