I think I started running a squad around fort mech in their lower 90's with TBM gear. It was near continuous killing, sometimes needing to rest to wait for repops. I used that time to organize inventories, adjust loot settings etc. There is a ton of trash that drops, trash that you may not want .. quest items and such. I like the first and third floors. The first floor has an abundance of mobs to attack and a vendor you can visit to sell the trash to, look for Inizen Nogglezop (Spells and Tomes) he's in the NE corner of the first floor. The mobs there are in their upper 70's.
Gyrospire Zeta, and Meldrath's Majestic Mansion can also be good, but you'll find higher level mobs there in their lower 80's. I haven't visited the steam factory yet, it might be worth a try. It's hard to estimate how much plat per hour you could get, not sure how quickly you'll be able to kill and if your group would face any difficulties.