I don't get why you think hEX won't handle it.Nope, Gr3 won't do. Since you want ot balance, you'll need to skip FastTrack. Without it gr3 won't be able to cope with bandwidth.
You need more power. 4011 will do for example
Google for "mikrotik bridge configuration with vlans".Thank you, yes my ISP is ok with that, I need to know how to set up the RB750, in order to achieve this.