Either do a left join and repeat all the post values for every tag or do two round-trip queries and manually join them in code.
JSON_ARRAYAGG. You’ll get the object all tidied up by database in one trip with no need to manipulate on the receiving client.
I recently tried MariaDB for a project and it was kinda neat, having only really messed with DynamoDB and 2012 era MsSQL. All the modern SQL languages support it, though MariaDB and MySQL don’t exactly follow the spec.