Hmm, so it seems the only "problem" load in this whole room is operating the four inner chandeliers on the same dimmer switch, since 1800W is 90% of the capacity of a 2000W dimmer, which seems to be the highest rating that a box mounted device is produced in.
I have seen some racks and cabinets which have 20A modules. Discounting 'E' for the time being, at the very least there will be six switched lines in this room: three for 'A' and one each for 'B', 'C' and 'D'. At the most, dividing the room into east and west halves brings it up to nine: 3 for 'A' and two each for 'B', 'C' and 'D'.
Nine switched lines could be accommodated for with fairly standard dimmers but that would be a mess. The switches are actually surface mounted and located beside the middle column on the south wall, which actually has a segment cut and hinged to swing open, exposing the subpanel. There isn't room for a rack or cabinet *there*. The simplest and least obtrusive installation I could think of would be to put three dimmers for the one chandelier on one side of the column and three dimmers for the other three groups of lighting on the other.
The piers with arches between them on this side of the room are essentially impenetrable and the construction between the floor and walls of this place is so strong and well done that it is an absolute nightmare to fish anything in this place. The half-columns are attached to the fronts of shallow, built out "channels", rectangular and hollow in cross-section. These were added onto the fronts of the piers many years after the building was built the centre column on the south wall is what accommodates the subpanel.
