Hey, I got it sorted out and thought I’d share the fix as it could help others dealing with the GA4 API. Here’s how I altered the ‘metricFilter’ section:

'metricFilter' => new FilterExpression([
                'filter' => new Filter([
                    'field_name' => 'eventCount',
                    'numeric_filter' => new FilterNumericFilter([
                        'operation' => FilterNumericFilterOperation::GREATER_THAN,
                        'value' => new NumericValue([
                            'int64_value'  =>  '10000'
                        ]),
                    ])
                ]),
            ]),

Turns out, you need to use a ‘NumericValue’ object for the ‘value’ field. Once I did that, it worked perfectly. Hope this helps!