For me one of the most flexible and mature way to knowledge base, tasks and notes is an org-mode.

I have two main workflows. The first one is task management. I have a lot of recurring tasks with tags, deadlines, schedules, etc. All of them are living in org-files in my Nextcloud. On Android I'm using orgzly-reviwed for sync via WebDAV, on my work I'm using organice (via WebDAV) as a "web-version" and also I'm editing my notes in emacs on my laptop (but actually any text editor could be used).

The second one is a knowledge base. I'm using org-roam locally (and with a localhost web server, built in into emacs) and orgnote for Android/Web + synchronization. My knowledge base is Zettelkasten-based.

Orgzly-reviewed: https://github.com/orgzly-revived/orgzly-android-revived

Organice: https://github.com/200ok-ch/organice

Orgnote: https://github.com/Artawower/orgnote

Orgnote provide a way to encrypt all notes by your own key/password. With orgzly I'm relying on Nextcloud encryption.

Logseq, kept up to date on all my devices with Syncthing

I’ve loved Obsidian since I started using it.

If I moved to OSS, it looks like Logseq would be closest.

I manage my entire life with Logseq. Syncing is done via github, since I have nothing to hide there. I would recommend setting up your own gitlab server instead.

I think I have a good one for you. You are probably familiar with Linear, a JIRA alternative. Well here's pretty much a FOSS clone: https://github.com/makeplane/plane

I self-host, and since the one thing I dont like is it's document library, I pair it with self-hosted Outline: https://github.com/outline/outline

Combined it's a really solid setup.

I use Joplin, there's desktop and mobile clients, sync with your own WebDAV server and you've a Kanban plugin. It also does checklists, tables and all the stuff people like. Stores everything in markdown so that's a win.

Many have mentioned org-mode. I used it for years but have moved on to todo.txt and markdown. I use syncthing to keep notes up to date on my phone and computer. I edit with whatever is available on my desktop and I use markor notes on my phone.

I think this setup only lacks the recurring tasks option. I think org-mode can do that but I use my calendar for that.

This is a highly personal topic so I'd suggest trying as many things as you can. Something will stick eventually.

Zim

Its amazing. I keep my whole life in there. Also has tons of plugins that probably meet your tasks needs.

I store it in an a encrypted luks volume that I upload to the cloud, so its zero knowledge on all the clouds.

Tasks.org for tasks and Joplin for notes. Both can be synced with various technologies.

Vim with markdown

I've always used joplin mobile for notes and vikunja for tasks and reminders. Vikunja has projects and labels for organizing tasks, and supports list, table, kanban, and a timeline view. There's a paid version (for corporations, I think) at https://vikunja.io/, but there are loads of other free to use instances. I personally use https://todo.projectsegfau.lt/. Edit: I wholeheartedly hope you at least try vikunja https://try.vikunja.io/, because it's one of my most used and beloved services.

I use the app opentask. Based on your criteria, it's exactly what you're looking for. It's all stored locally on your phone, it has the ability to set recurring tasks, and you can create categories of preset and custom tasks. Deadlines can be set as far into the future as you need, down to the minute. It's available on F-Droid.

LogSec is really nice and flexible.

Foam is pretty good https://foambubble.github.io/foam/

Zim Wiki https://zim-wiki.org/

Desktop wiki, saving to .md text files, can commit to git repo and has basic task handling.

Perfect for me.

Zim can be used to:

Keep an archive of notes
Keep a daily or weekly journal
Take notes during meetings or lectures
Organize task lists
Draft blog entries and emails
Do brainstorming

Right now mainly Nextcloud for task and kanban with Jtx and Deck on android.

For my knowledge markdown files edited with Quillpad, Zettlr, Tangent, Nextcloud …

That's not zero-knowledge but that's work for me and I feel that secure enough in my case for my daily life. :)

Org mode on my laptop, don't see any need for anything else.

I just use Zettlr (a markdown editor optimized for writing research papers). I wish it wasn't an electron app, as it's paggy as hell sometimes on Linux, but it's the best balance I've found between features, ease of use, and stability.

@makeasnek emacs + denote and org mode

Taskwarrior

I've never found a use for any of this. I only utilise a calendar. How complicated are your people's lives?

I’ve been using todo.txt for tasks for about a month now—it’s dead simple, supports all the bells and whistles you mentioned; and, with the topydo CLI, you can very easily make yourself a kanban interface using its columns UI. I sync the files with my iPhone and use Todooo on iOS, which works beautifully.

As for notes, I just write simple text files with my favorite editor. Maintaining complex systems of interconnected notes, I’ve found, most often does not pay off for the enormous time investment required (some specific use cases aside); tags, links, etc. I have all found to be superfluous—any kind of grep integration in the editor is all that’s needed for finding things. I write in either markdown or Typst, because basic Typst is essentially the same as markdown anyway, and because I’ve found it very useful to keep notes in the same format I write longer-form documents in.

I use SuperProductivity. It works really well. You can keep track of how much time you spend on individual tasks and I sync it to my nas server so it's synced on my phone, desktop, laptop