Ideally it should be simple, free & open source, and easy to self-host. Ideally it should have nothing to do with Atlassian.
Is there something that you have tried and would recommend? Something else that you tried but found to not work too well? Thanks for sharing your experience!
Here's an example with a _simple_ kanban board. Maybe it helps:
- PlantUML source: https://gist.github.com/igponce/0b7372621c8df7d503df80760ec1...
- Generated SVG: https://gist.github.com/igponce/f2c079536391be702f6060cfffa0...
I make PlantUML diagrams in VSCode. There is a plug in for plantuml that helps with the preview and exporting diagrams
Also (Offtopic): Not open source, but if your company already has a Microsoft Project license, you can use it like trello for sprints (and make the "done" tasks fill the gannt chart - which is still usefull for planning and resources and takes into account holidays, etc).
Minimalistic UI that gets out of your way, easy to run in Docker, fast, lightweight. I think it hits all your requirements.
They even have a Trello comparison page: https://asana.com/compare/asana-vs-trello
I'm curious, what do you have against Atlassian?