Hidden Delayed node for backup


I have a mongo version 4.2

I am considering using a hiden-delayed node as a backup strategy, instead of my current mongodump, as my data becomes very large, and unfortuantly my budget currently doen not allow me for ops manager.

I have a few quetions regarding this backup strategy:

  1. In case of a need to restore from the delayed node, I need to use mongorestore - so the amount of time needed in such a case id very long, jsut as restoring form my mongodump - Am I right?

  2. In case I configure the node to be delayed by 12 hours - how large should my oplog be in order to ensure no entry is lost (as oplog is overwrittten)? is there a formula for that?

  3. My current setup is 5 nodes - 1 primary and 1 secondary in a local site, and 2 secondaries and 1 arbiter in a remote site.
    I understand that the hidden node is a non- voting node. Does that mean that even though I have an even number of nodes, there will always be a majority for a Primary election?