Alter the wp-json/wc/store/v1/products/<ID> response so that it includes 'more complete' information about variations.

woo-store-api.full-variation-data.php

<?php

/**
 * Modify /wp-json/wc/store/v1/products/<ID> so that it returns more complete data about each variation.
 *
 * Not extensively tested, just captures an idea about how we might do this.
 */
add_filter( 'rest_request_after_callbacks', function ( $response, $handler, $request ) {
	// If this is not a Store API product request, do not interfere.
	if ( ! str_starts_with( $request->get_route(), '/wc/store/v1/products/' ) || $request->get_method() !== 'GET' ) {
		return $response;
	}

	// Determine the product ID, or bail if it has not been provided.
	$params = $request->get_url_params();

	if ( ! is_array( $params ) || empty( $params['id'] ) ) {
		return $response;
	}

	$product_id = absint( $params['id'] );

	// @todo we know the ID, so let's confirm it relates to a variable product before going any further.
	
	// Make an internal request to load complete variation data.
	$sub_request  = new \WP_REST_Request( 'GET', "/wc/v3/products/{$product_id}/variations" );
	$sub_response = rest_do_request( $sub_request );

	if ( 200 === $sub_response->get_status() ) {
		$full_variation_data = (array) $sub_response->get_data();
	} else {
		return $response;
	}

	// Add additional information to the response. In this case, we are overriding
	// the existing `variations` property but it may be preferable to add this 
	// information as a new property, to avoid surprises for other API consumers.
	$payload = $response->get_data();
	$payload['variations'] = $full_variation_data;
	$response->set_data( $payload );

	return $response;
}, 10, 3 );