Comparing Decipad and Notion (both of which I have limited experience with), I find collaboration to be similar. In both cases, it's real-time and seamless. Notion is more structured (and IMHO more clumsy) while Decipad is more free-form, but ultimately collaboration is similar. Versioning in both cases is a timeline-based version history. I've never used Observable.
> Todoist: Great until I realized I was gaming the points system instead of doing actual work. Turns out completing “drink water” 8 times a day doesn’t make you productive.
OP was drinking 2+ liters of water per day. It may not be productive work-wise, but it's productive health-wise.
2023 was the first year I gamed exclusively on Linux according to Steam's year in review, including some of this year's titles. Most of that was on the Steam Deck or on a virtual machine with GPU passthrough running Bazzite. It is really well made.