How to create new Relative Date Filter?
Certain ones can work by just adding option values, but I don't think everything. For example @andy-clark you can do prior 6 months as "ending_6.month" which is supported by the default clause here https://github.com/civicrm/civicrm-core/blob/5.20.2/CRM/Utils/Date.php#L1660

Take a look through that function to get an idea of what works, in particular the default clauses in each section.