The War Steeds is not locked... you get credits for each win of your actual chosen City State, joining as freelancer, which is for all, will not ruin that.

If you feel like it takes to long to get the mounts from PvP, one thing, get good with the job you like to play, so you do a difference and secondly try and work with the alliances in there and not against them... going in solo somewhere only rewards the opponents if they are just a little bit organized.