Mail on my Mac Pro running the latest update of High Sierra (newest OS I can use) keeps crashing when I use "Select All" to empty the trash
When you rebuilt the mailbox did you also get it to re-index? 

Quit mail, then ~/Library/Mail/V6/MailData/ delete all the Envelope Index files [duplicate to the desktop first for safety] &amp; re-launch Mail. 

The rebuild may take some time