Here below is a default objectivecycle.cfg file that comes along with ET. Check it out and tweak it with your own maps. I see as you tried to do it already, but the cycle was incorrect and it has to return to start from the last map to work.
set d1 "set g_gametype 2 ; map oasis ; set nextmap vstr d2"
set d2 "set g_gametype 2 ; map battery ; set nextmap vstr d3"
set d3 "set g_gametype 2 ; map goldrush ; set nextmap vstr d4"
set d4 "set g_gametype 2 ; map radar ; set nextmap vstr d5"
set d5 "set g_gametype 2 ; map railgun ; set nextmap vstr d6"
set d6 "set g_gametype 2 ; map fueldump ; set nextmap vstr d1"
vstr d1