wondering if I'm better off using that from Open Street Maps or just using the default that comes with the 520 plus?
You can place multiple maps on the unit, just make sure they have a unique file name. The approach outlined for the 520 was because it did not support having maps so people were tricking the system by using the one map name supported by the 520.
To change maps you need to go
settings>activity profiles>(select your profile)>navigation>map>map information
On a related note, since these maps are openstreet, can we share the ones that came with our devices