What's the low-down on MariaDB JSON support? Does MariaDB ever plan to have full JSON support or is it just going to be an alias forever?

It's a bummer to set constraints definition in the table just to make it work properly. Application-level migration libraries (like Phinx) doesn't support it so you have to do extra work which defeats the purpose of migration libraries.
No plans that I know about.

Regarding constraints, I assume you mean for JSON validation: See Changes and Improvements in MariaDB 10.4:  


  The JSON_VALID function is automatically used as a CHECK constraint
  for the JSON data type alias in order to ensure that a valid json
  document is inserted (MDEV-13916)


... so you don't have to set the constraint explicitly.