I use Nextcloud Notes and Tasks extensively.

Notes is kind of bare-bones compared to Carnet, which is more like Google Keep, but it’s fast, syncs with its own Android app, and stores notes as regular files in your Nextcloud folder so you can use any text editor with them.

Tasks hooks into the calendar system and can sync with anything that supports CalDAV. I use Davx5 to sync it (along with my calendars and contacts) to my phone, where I use OpenTasks to actually manage my to-do list. The only problem I have with it is that it doesn’t support recurring tasks very well. I’ve sort of managed to work around that by syncing with Thunderbird, which lets me create recurring tasks in the underlying calendar data.

Something like Deck?

Deck is a kanban style organization tool aimed at personal planning and project organization for teams integrated with Nextcloud.

I personally like the Kanban-style overview for managing tasks more than a simple To do list.

I use tasks and collective mostly. I was using notes but had issues with syncing so I stopped using them