Hi @mrwindmills
First of all, good choice on making the switch to PostgreSQL 
When it comes to migrating between NoSQL and SQL the biggest issue is going to be remapping all your relations due to the difference in the ID type (Mongo uses an ObjectID and PG uses an integer). Currently we don’t have any supporting tools to do this and will likely require a custom migration script to be built.
As to the schema, Strapi will initialize the PG DB schema on it’s first boot, after that it’s simply migrating data and rebuilding the relations, the downside to the relations mapping however is they are handled differently between the two.
With SQL on certain relations you have a middle table used to connect the two model tables, on MongoDB this isn’t usually the case. I would advise you take a dump of your entire mongo database into a format that you can import into PG (skipping the ObjectIDs during the import, let PG recreate new IDs) and try do remap them.
Unfortunately this will not be an easy process, and I would recommend going forward, using PG locally as well (or even just SQLite)