I am thinking Backup and Restore is not an option as there is no backward compatibility for this. I have not tried it.
Looks like SQL 2019 has not done any changes to age old Import Export wizard. This can be used to load data from other databases and servers. That being said, the relations between tables will not be copied over if this is used.
I would try the Generate Script option and select schema and data if you would want to move data and schema objects along with any constraints and indices for few tables. SSIS packages is a great option to schedule it leave it running if your database is huge. Have you tried these options?