Bulk Object Relationships add/set?

No, bulk insert/upsert works only with primary objects and doesn’t do anything with related objects

The bulk save/upsert returns a collection of objectId values. You can pass that collection into the children connector of the Add/Set Object Relations block:
UI Builder - ConsoleDemo - Backendless - Google Chrome 2022-03-22 17.22.44

Regards,
Mark