I would like to see the score and the score calculations in my client. There are some plugins for Thunderbird (ex: spam-scores), or if needed to develop custom plugins.

Outgoing score is irrelevant. The receiver server will not take it in their calculations. 

BTW, modifying subject with [POSSIBLE SPAM] is also "bad practice" since there is client side DKIM validation, or if I validate the message (for any reason), it is modified and DKIM signature will be always wrong. It is better to add custom X- headers and the e-mail client developers to consider to allow to the users to customise which header to display.