HEllo @IGnatiusTFoobar its wonderful when a lead developer of a project in Question is available on the community to answer and assist on the project. Welcome
In relation to citadel as a Module, @stephdl is at the moment our modules champion, and did a great Job mentoring me into module development, mAybe he could tip you on one or two.
With regards to Deeper integration i think @davidep from the @dev_team would be better placed on this.
IS it possible for you to share a docker compose file for Citadel this would help understand the inner working of the services required for the Module.
Following the docker run instructions from the link posted,
is it possible to make use of these instructions here: NS8-Generic Containers-: Install Uptime-kuma - Howto - NethServer Community
To make citadel working or installed on NS8.
All Modules BEgin with this: NethServer/ns8-kickstart: NethServer 8 module template (github.com)
You might want to take a look at this module here: NethServer/ns8-mail: NS8 Mail module with SMTP, IMAP, Spam/Virus filter (github.com)
as well as this: NethServer/ns8-webtop (github.com)
they would properly give you an idea onto How Modules, of similar nature to citadel would work.
Sometimes we use existing docker images, sometime we build a new image from the module, directly from source, Depends on complexities required to handle the installation
from some previous discussions her eon the forum, its not possible to have 2 mail instances running on the same Node, My presumption is that, with citadel a a module, a user would again not be using the NS mail module implementation.
Since citadel has its own packages for handling mail, HWo know, Maybe similar to how the mail module is implemented, it could be seprated from the instance, and then other Mail interfaces like Sogo or webtop could take advanage of the same.
this is way beyond my scope of knowledge
Here are the Dev manuals Home | NS8 dev manual (nethserver.github.io)
Hopefully the given information would help you try to mirror what could be possible of a citadel module(app) for NS8