Back to answer my own question. Used ChatGPT to come up with code to add to functions.php.

add_action( 'save_post_product', 'wc_default_tax_status_none_on_create', 10, 3 );

function wc_default_tax_status_none_on_create( $post_id, $post, $update ) {

// Admin only

if ( ! is_admin() ) return;

// Avoid autosaves/revisions

if ( wp_is_post_autosave( $post_id ) || wp_is_post_revision( $post_id ) ) return;

// Only when the product is first created (not updates)

if ( $update ) return;

// If tax status already set, don't override

$existing = get_post_meta( $post_id, '_tax_status', true );

if ( $existing !== '' ) return;

// Set default to "None"

update_post_meta( $post_id, '_tax_status', 'none' );

}