There is also kinda similar open source software plane
Don't forget the highly customizable and themeable Kanboard, made by the same people behind Miniflux. I will check out plane, though. Looks neat!
Story time! (YMMV as they say)
I decided I wanted to self host an app like this just a few weeks ago. I started with Focalboard but just could not get it running as a personal server instance on Docker. (This could entirely be on me as I am very much still learning) I got confused in the whole what's Mattermost / what's still part of Focalboard talk, and I wanted to use a Docker compost file but they don't have one on this page. I got an instance running anyway, but the site wasn't responding when I tried to load it in my browser. I've made this work now for about 8 other apps but just could not find the root cause with this one.
I ended up installing and loving Planka though. For my extremely simple use case, it's just right. The docker installation was dead simple and it 'just works' for me.
Nextcloud Deck has been pretty solid for my usage.
I still like https://vikunja.io/ more because they has Gantt chart which is a super rare feature for modern project management sw.
Sounds cool, however don't forget this is under MatterMost Licensing:
A source available license gives access to source code, but places restrictions on its use. The Mattermost Source Available License allows free-of-charge and unrestricted use of the source code in development and testing environments, but requires a valid Mattermost Enterprise Edition License in a production environment.