Load an ACF field's options dynamically via PHP. For more info see https://philkurth.com.au/tips/load-an-acf-field's-options-dynamically-via-php/

acf-load-field-options-example.php

<?php

add_filter( 'acf/load_field/name=countries', function ( $field ) {

	$field['choices'] = [
		'au' => 'Australia',
		'nz' => 'New Zealand',
		'gb' => 'United Kingdom',
		'us' => 'United States',
		// …
	];

	return $field;
} );