Yeah i get that - there are moments when you either have plenty of time to get into position (the start) or when you're in a base defence when a mech like the King Crab can be very effective.
TDR-5SS is the front taking hits while taking there torsos away. BJ-1X is fast enough to support if you die early in a push and you friends are still alive. BLR is fast enough Lurm 2nd line pushing and can do 800-1300 damage on the good battles.
But unfortunately you can't bring four 30t... too low on tonnage. I suppose PGI anticipated how OP four Urbies would be and set the rules in order to prevent such exploit.
Usually I play to defend: A high damage assault with tons of ammo (currently a KGC with 4x AC5) A medium sniper or LRM boat (currently CN9 LL sniper, to be swapped to an Enforcer) A heavy sniper or LRM boat with nothing but tons of ammo and enough DHS to chain fire til the end (currently a LRM cat) A light sniper for some mid-long range damage without taking up tonnage. (currently commando with 2x LL) LRM boat for first drop to deal damage before and during the gate bash. LL or sniper for basic damage dealing. Assault for more tanky things or most likely as a turret for damage at choke points. Light for clean up.