The first problem you have is deciding how to dissipate 945W of power in your chandelier. The efficiency of the LEDs at converting this power to light is inconsequential since light itself will heat any target it strikes, and I assume the light from individual LEDs will hit other parts of the chandelier structure.
The problem of adding switching regulators will only make the equation worse by 20% (most switching supplies are above 80% efficient). The total power dissipated in and around your device is therefore likely about 1.2kW.
The best solution would appear to be to raise the DC supply to the maximum you can handle at your point of connection to the AC mains. Perhaps direct rectification of the mains (120V??) would be viable. This would provide about 160V DC and require only 7.5A.
Then you can distribute 160V - 12V DC-DC convertors within your structure so the 12V lines are as short as possible.
Note: you don't say how many LEDs there are or give technical details on them. Are you using hundreds of LEDs or only a few large LED preconfigured in strips(say 50W)?
To provide any real advice you'd need to supply much more detail such as:
Why 12V to drive the LEDs? Why are you not connecting multiple LEDs (or even LED strips) in series to reduce current flow in the structure? For example: You show in your diagram LED strips, so assuming these are just plain LED strips at 45W they will draw about 3.73A per strip. You could connect 7 strips in series so requiring about 84V @3.75A. You might be required to match the strips against light output and current, but that process is simple. Now you have only 3 * 3.75A distribution points. A much more manageable current configuration.
Do you need to modulate the lighting? This includes brightness control and generation of patterns. Do you need PWM control, or data streams to individual LEDs?
What is the maximum temperature your plastic can tolerate without losing significant strength?
- What airflow restrictions exist in the structure? Why can you not provide air into the structure to dissipate heat.
Since based on your comments each strip is composed of 220 LEDs, I'd suggest you cut these strips into 10 sections of 22 LEDs each and wire these smaller sections in series. That will raise the voltage requirement to 120V @45W, or about 375mA per strip total.
Since you have 21 strips in all this means that the total current requirement is 21 * 0.04 --> about 7.875A. That can be done with a quite reasonable wire size.
I assume you will want your installation to meet NEC code as much as possible, so you could do no better than using the NEC 310-15(B) to select your wire size.
To keep the wire temperature well below 80degC you could select a wire capacity say double your requirement. If we look for wire from the chart then you could use 14G as a very conservative selection. This would support more than twice what you require and this would keep the cable temperature as low as possible.
The voltage drop for these wires is usually calculated at about 2-3% over the run length (you can look up details), so you'd expect to have a voltage drop @8A of only 1% with this underrating and wire temperatures < 50degC.
Raising the delivery voltage to 120V does of course mean a more specialized DC-DC convertor both at the mains entry point and within the chandelier, which might raise difficulties for you if you just want to select existing product.