lol yeh they are all elf elites and everyone of them won initiative? or maybe not. Or I misty stepped as a bonus action too and they are still at 30 foot range..And they moved, dashed and misty stepped to you as a bonus action.
I started to say well if we are going humanoids maybe Drow? a Shadowblade is going drip with mobility with a 60 foot teleport though he is CR 11. (but this all highly depends on the specific enemies and initiatives rolled etc so we really cannot be sure about the results)
Depending on the enemies at this level they would not necessarily even know the value of running next to the players. Some will think hey I can definitely take this and be right they might.
Some of that assumes that all moves happen before the archer got off his prone or menacing attack or before the web spell drops in place potentially slowing many of them ... and so on, but regardless it does make a somewhat interesting fight (having an unpredictable element involved) with all participating and that is a point in your favor. If these things weren't all useful on their own and only helped keep the t-rex on the enemy that would be different because we could declare them part of the sunk cost of the polymorph (but slowing enemies and getting back to better range and so on are useful anyway)