First of all, thank you for the awesome work you've done !
I installed Bitnami MongoDB with Replication in my gcp project and it's really well done. However I noticed that you use ext4 file system for the data disk while using WiredTiger storage engine. Could you use an XFS file system instead for the data disk to comply with the official mongodb documentation and I quote :
"When running MongoDB in production on Linux, you should use Linux kernel version 2.6.36 or later, with either the XFS or EXT4 filesystem. If possible, use XFS as it generally performs better with MongoDB.
With the WiredTiger storage engine, use of XFS is strongly recommended to avoid performance issues that may occur when using EXT4 with WiredTiger."
Here is the link : https://docs.mongodb.com/v3.4/administration/production-notes/#mongodb-on-linux