Found the issue! It was a type error in my Course.php model:

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class Course extends Model
{
    use HasFactory;

    protected $fillable = [
        'classdescr',
        'classmeetinginfo',
        'associatedclass',
        'classattributes',
        'prereqs',
        'description',
        'instructor',
        'reqsfulfilled',
        'quartersoffered',
        'gradingpercent',
        'difficulty',
        'hrsperwk',
        'subject',
    ];

    protected $casts = [
        'classdescr' => 'json',
        'classmeetinginfo' => 'json',
        'associatedclass' => 'json',
        'classattributes' => 'json',
        'prereqs' => 'json',
        'reqsfulfilled' => 'json',
        'quartersoffered' => 'json',
        'gradingpercent' => 'json',
        'difficulty' => 'float', // was 'decimal'
        'hrsperwk' => 'integer', 
    ];
}