Oh OK I see now.

The filter modifies the JSON-API responses. The JSON-API data is used by the dynamic UIs.

If you are going to use template=”static”, then yes, the best way to do this is to modify the template.

Instead of

if ( isset( $adapters[ $atts['symbol'] ] ) ) {
        $balance_str = sprintf( $adapters[ $atts['symbol'] ]->get_sprintf(), $balance );
} else {
        $balance_str = sprintf( "$atts[symbol] %01.8f", $balance );
}

You could have something like:

$balance_str = sprintf( "%01.8f", $balance );

You should place your modified copy of the template under your theme directory, at /templates/wallets/balance-static.php, as described here.

Hope this helps, and apologies for the confusion.