Have you looked at PM2? Digitalocean has published quite nice tutorial.
https://www.digitalocean.com/community/tutorials/how-to-set-up-a-node-js-application-for-production-on-centos-7

I looked at https://forum.qt.io/ categories/subcategories layout and it is very convenient, especially when you have many subcategories. Any tips on how to implement similar solution?

Transifex has CLI client - so you don't need to manually download all translation files.
https://docs.transifex.com/client/introduction

Yes, NodeBB uses ImageMagick.

https://github.com/NodeBB/NodeBB/blob/7d0eb9afaa0ac51b814cfaf733d98c8259c64742/src/image.js

Extract all data for one user:
example on this forum: https://community.nodebb.org/api/user/egut

Deleting a user:
Write API plugin
https://github.com/NodeBB/nodebb-plugin-write-api

@pichalite Problem is in Persona theme, not NodeBB itself.

Not that I know of. Aren't they self-describing?
Could you explain where exactly are you stuck?