Yeah, it's not perfect. I've noticed that when making a task list/checkbox note that you can't delete the line. Sure you can go and edit the contents of that line, but even if you delete all text, the line with a checkbox remains. Kind of annoying, particularly when I accidentally create another checkbox and have nothing to put in it. it's just an empty line and checkbox sitting there mocking me. You can still mark it complete of course, but it kinda makes my eye twitch. :)
Just to throw my own experience in the mix. I tried the AIO and standard versions of Nextcloud and found them to be flakey and slow. But I felt compelled to keep trying. That's when I found NextcloudPi. I've installed it on a Pi4 running from an external SSD and it's been rock solid. I believe that version is no longer in development though and I primarily use it as a sync platform for various apps rather than using the web apps directly.
And just to be contrary, have you looked at Seafile? It's stupid fast and stable but some features are hidden behind a paywall if you have more than 3 users (community vs pro). Their documentation is poor, and the data is stored in Git-lik chunks on the server. All of which can be a deal breaker for some. The external storage feature works, but for a newb like me, it was a bear to get running.
Yes. I have a Pi4 running NextcloudPi image on it. I sync docs, pics, even backup my Obsidian vault. It's worked really well for Quillpad in my experience. On desktop I use Iotas (Linux) if I need to update from that instead of my phone.
Quillpad is the closest I've found. It's simple markdown files. It can sync with Nextcloud as well. I use it for any short note or lists. Long form stuff including journal, I use Obsidian (not open source)
I was looking for a journaling app that didn't have vendor locking, or required some weird export dump that messed your formatting and folders up. That lead me to Markdown and Obsidian. I love it. And when I die, that shit will still be readable by any basic text editor.
Memos is self hostable and is "cross platform" by nature of being web-based only. There is a 3rd party mobile app MoeMemos but it doesn't add anything special over the quite excellent progressive web app for plain Memos. Of course you can't use it offline since it's web-based. But I have an always on VPN connection between my phone and my server so home so it's fine.
Notesnook is recently open source, but as of yet not self hostable. It is on the roadmap though. This one is privacy/security oriented and has native apps for just about everything as well as a web interface.
Quillpad is the closest interface-wise to Keep, but it can only sync with Nextcloud and I can't run that beast on my old hardware. Too clunky and slow.
I've been on this hunt for awhile but I realized that I use Keep differently than other folks on the same journey. It's mostly a list focused service for me. Sometimes with check boxes, sometimes not. Most of the FOSS not taking apps can use some markdown, but that is a bear to use on mobile without a quick way to inject a checkbox. Memos has a button for a few formatting items on each "post" and thankfully one is the Markdown checkbox shortcut.
For notes, personal knowledge management, and everything else I use and love Obsidian.
Yeah, it's not perfect. I've noticed that when making a task list/checkbox note that you can't delete the line. Sure you can go and edit the contents of that line, but even if you delete all text, the line with a checkbox remains. Kind of annoying, particularly when I accidentally create another checkbox and have nothing to put in it. it's just an empty line and checkbox sitting there mocking me. You can still mark it complete of course, but it kinda makes my eye twitch. :)