A while ago, I started keeping a personal library/journal/etc. using Logseq. I could fire up Logseq in any browser on the planet, connect to my notes, and jot down whatever idea I had in the moment, all in a FOSS journal that stored my notes in plaintext markdown.
Then ... I don't know what happened, but 100% of their effort went into building an app, which then required them to build a (paid, proprietary) sync service, all rather than just releasing a self-hosted build of the web interface so I could spin up my own note-taking server. (Please don't suggest alternatives; I've probably tried them all.) To "preserve privacy" and promote "local first", I had to download an app and rely on a closed-source backend to do something I could trivially accomplish on my own. If my platform doesn't support the app, no notes, unless I rely on the increasingly unmaintained web "demo" that does exactly 100% of what I need from the service, despite dozens of features missing compared to the app version.
But the kicker is that I cannot install things on my work computer. At all. Not portable apps, nothing. I will get a phone call from infosec if I even try, because we are a heavily regulated company. So if I have a bright idea at work, a thought I want to preserve, find a good article, etc., I have to go to another device. I have to interrupt my workflow, change my focus completely, and, probably, lose half of what I wanted to capture.
The thing is, I don't think they're data farming. I think they're running a really good project! Users were begging for an app. "When are you going to release an app?" was a common question forever, because a whole generation of dingleberries cannot be bothered to go to a website that does the same thing, faster, and better than any app.
Logseq is really, really close. It's basically a page I can start writing on, forcing minimal organization through bullets but otherwise freeform. Backlinking, plugins (meh), plain markdown. It's just so good. It doesn't require me to do anything other than write. It used to be entirely browser-based, syncing through a github repository. They could have released a self-hostable version of that and I would have been over the moon. Or, alternately, a self-hostable version with non-local storage so I could store my notes on a notes server I control. But they went with the app + sync service route. Understandable but sad.
So I just rolled my own sync through a git server and it works fine (other than iOS, which requires a maddening setup, but that's not logseq's fault).
I looked at Zettlr once or twice (thank you for mentioning it). Obsidian makes me crazy with all the UI fiddly bits and configuration. I tried. Oh how I tried. But it just didn't work with my brain. (It's the exact same reaction I have to KDE -- there's just TOO MUCH and it sets me off in unproductive directions, and that's not a criticism of either project as such.)