Obsidian

• text files stored in markdown on your device
• no premium service to unlock features. You can buy sync and publishing if you want to, but there are no nags to purchase them.
• you can roll your own sync solution you wanted to. Its just text files. Same goes with publishing.
• community plugins if you want to extend it
• desktop clients and ios clients
• free forever

https://obsidian.md/

There is Google Keep.

But if you want to degoogle, well I'm reading the thread. I've heard of Joplin (spelling?).

Obsidian, Hands down.

Markor. It's as simple as it gets, but can scale up if you decide to later

Markor.

Then I also add my notes / docs folder to syncthing, and have it on every device.

SimpleNote

This is what I use across all my devices

https://simplenote.com/

I use signal mesagaing app. They have a note to self feature thats awesome.

I used to use Keep Notes by Google. Did the job pretty well but obviously there should be better FOSS & privacy-respecting apps: NoNonesense Notes!

https://www.f-droid.org/en/packages/com.simplemobiletools.notes.pro/

This and others by simple are exactly that.

Notes. Gallery.Dialer. keyboard. Clock.

No ads. Absolutely zero unnecessary permissions. No telemetry or sneaky internet connections.

Obsidian.md. It runs on markdown files, so even if you decide you don't like Obsidian, you've still got all the files. It's super easy to sync with SyncThing or probably Google Drive or whatever. It's free, and the are a ton of plugins for it. Peri sure there's a Lemmy community for it around, too.

I've been using Notally for a while and I think it checks all your boxes.

Quillnote + Signal's "note to self"

You can find Quillnote in both Droid and Google's Play Store.

Quillpad is like an open source Google Keep and you can use Nextcloud to sync it all.

Ideally, I'd want a way to import my Google Keep lists into it then I could bin Keep for good.

.txt file + syncthing

For true rebels: pen and paper

One option is Notally on F-droid and on the Google Play Store. It's FOSS (Free and Open Source Software) and it has all of the features you mentioned. I use it as my everyday note taking app and I have zero complaints so far.

There's Safe Notes on F-Droid as well, which is pretty much identical to Notally feature-wise, but your notes are encrypted (locally).

Obsidian is kewl cloud sync aint free though.

I found Obsidian good, and Markor the best, on Android.

Notion has been pretty great so far; I recently moved there from Evernote. Only downside (for me) is that it's based on electron.

I use OneNote usually, easy formating and works well on PC and mobile. Notion is cool too, but has a lot more features than a simple note taking app.

ColorNote

It meets every item on your list.

I've always used ColorNote. It does have several advanced features but they're all optional. It's been updated for 5 years now and basically hasn't changed. Still free, no ads, easy to use. A rare exception on the Play Store. Whole app only 4Mb.

Apps:

• Zettel notes https://play.google.com/store/apps/details?id=org.eu.thedoc.zettelnotes
• Markor https://www.f-droid.org/packages/net.gsantner.markor/

Self-hosted:

• https://silverbullet.md
• Memos https://github.com/usememos/memos

Kvaesitso launcher also has built in note widget that can be linked to same local file as in an app. https://github.com/MM2-0/Kvaesitso

Element for Matrix. I know weird, but that's how I like it

Google keep works fine for me.

According to this article, and others I've seen pen and paper is still best.

https://www.digitaltrends.com/features/handwritten-vs-keyboard-notes-science/#:~:text=Another%20study%20from%20the%20same%20year%20found%20that,in%20a%20more%20comprehensive%20understanding%20of%20the%20content.

ColourNote has been the best one I've found.

Obsidian.... forever

Great, now how about a sketching app that follows all the same criteria that OP mentioned. Something for quick and dirty sketches. Nothing fancy. One that can do sketches and notes would be even better.