Just using Nodebb not a long time.
- Backup is not convenient currently and you can't do what you say for now.
- Yes, just install the plugin.
- Installing NodeBB on the Cloud
- Nodebb installation is easy and fast more than Discourse, you don't have to build a Docker image.
- I have not an upgrade yet, newest version I was installed. Also, you can go https://docs.nodebb.org/configuring/upgrade/
- Third part Email service is optional and you easy to config email on Nodebb.