I'm frankly not a fan of the monolithic NextCloud, but the "Decks" feature has good UX and a mobile app on Android, which pretty much nothing else in the open source community has pulled off.
That's what I'm switched to, especially since I already use Nextcloud as a cloud backend for my phone.
From the kanban suggestions, I tried kanboard in the past, but really disliked the mobile experience. In this regard, Deck is much better, and it has at least two ways to access the boards; one is the Nextcloud Deck companion app, and the other is the jtx board, which stores its tickets in a way that they can be synced with CalDav. So by using Nextcloud Deck, one is not even locked in into one application / provider.
I suggest to actively keep up with Nextcloud project. They have a lot of things going on, and add nice things over time. "Notes" for example is also useful.
From the kanban suggestions, I tried kanboard in the past, but really disliked the mobile experience. In this regard, Deck is much better, and it has at least two ways to access the boards; one is the Nextcloud Deck companion app, and the other is the jtx board, which stores its tickets in a way that they can be synced with CalDav. So by using Nextcloud Deck, one is not even locked in into one application / provider.