I’m curious if you have any updates on this?
I’ve had jobs to treat similar large commercial spaces and worked with architects and acoustical engineers to develop solutions to similar problems.
Basically what you’ve been told for RT60 is correct. The value won’t change with placement. All that matters is the number of panels. However that is not true of things like flutter echo or slap echo problems. You could treat only two entire walls in that room and possibly reduce the RT60 to a very low number but still have reflective surfaces causing problems. Typically flutter echo is easily fixed but you do need to identify the offending surfaces and treat one of them. It’s not always obvious either. The general rule is that it’s caused by two parallel surfaces but in fact it can be caused by near by perpendicular surfaces. Soffits for instance case cause these.
In my own work on rooms this large I usually incorporate reflector panels to help reflect sound toward absorbers and away from musicians and conductor. There are companies who specialize in these but they are typically made to order.
You will notice this is often turned into sculptures in modern Lobbies. I was really impressed by the giant diffuser, reflector sculpture I saw in an SF google building.
Diffusers can be used instead of reflectors but they need to be scaled appropriately. I think what you have is just too small for the space to effectively resolve these clarity issues.
You may also consider measuring a balloon pop impulse instead of a speaker. Unless you have an Omni speaker it can give you false results on clarity and even RT60 to some extent. Especially in large rooms
