I’m not quite sure, but this sounds a lot like issue intake rather than collaborate.
In anycase, we’ve used OS ticket for many workflow tasts from IT, HR, and facility management.
I believe they offer an on prem version as well as hosted.
https://osticket.com/editions/
Introducing a new OS, a new container system, and a new programming language into the environment won’t kill you.
Definitely easier and more secure than using an existing SaaS product right😆
Have a look here:
https://www.yetanotherforum.net/
Free, open source, all in ASP.NET, just unzip, setup configs, and install through wizard. Then mess with configs to get it to look just like you want it. If you need to modify something just load the source into Visual Studio and hack away…
Just like phpBB, or vBulletin, or Xenforo, or SMF, they are all worth a look if all you want is a simple forum, easily configurable… I have setup entire forums from scratch with vBulletin and SMF in the past in a few hours/days…