Is this a recommended configuration?
It's OK to have replication job use backup copy data as its source.
RPO won't be that great, but some of our customers are using this strategy and quite happy with it.
My main question is, how do I tell the replication job when to start?
Estimate approximate time when backup copy job typically ends, add some overhead (an hour or so) and set it as a replication job start time.
Is there a better way of doing this?
Potentially you can set a script (that executes a replication job) as a post backup job activity. This way, once the backup copy sync interval expires, post-activity will trigger replication job.