Flatten Eloquent Query results for CSV export

I want to flatten an eloquent query result which includes a number of many to many relationships using pivot tables. So all the returned data is in a single dimension, with the for export to CSV (using maatwebsite/excel).

Here's what I have in the Export class:

<?php

namespace App\Exports;

use App\Models\Chit;
use Maatwebsite\Excel\Concerns\FromCollection;
use Maatwebsite\Excel\Concerns\WithHeadings;

class ChitsExport implements FromCollection, WithHeadings {
    private $shootId;

    public function setShootId($shootId) {
        $this->shootId = $shootId;
    }

    public function collection() {
        if ($this->shootId) {
            return Chit::with('Negotiables', 'faaSupplementaryCharges', 'pactSupplementaryCharges')
                   ->where('shoot_id', $this->shootId)->get();
        }
        return null;
    }

    public function headings(): array {
        return array_keys($this->collection()->first()->toArray());
    }
}

I want the output format to be something like:

ID, Name,NegotableName1,NegotiableName2,SupplementaryCharge1....

Where the column names for the Negotiables & Supplementary charges would be the aggregated from the join tables with the values of each of those fields shown in the sheet.

So a simple example might be:

Name,Change Of Clothes,Working With Animals
Fred,20,40
Joe,,40
Melissa,20,40
Catherine,20,

"Change of clothes" & "Working with animals" would be the 'Name' column value from the 'Negotiables' table and the values would be the corresponding 'Amount' field in the Negotiables table.

I can't for the life of me work out how to do this, any ideas?