I’ve found the PWA adequate for my phone usage. I found a custom CSS that is sort of a Gruvbox that I really like.

Miniflux is great. I use Wallabag as my read it later app and selfhost both on a cheap VPS. They’re tightly integrated but Miniflux supports several other integrations

Honestly it seems like Obsidian is the one matching most of your criteria. $4/mo isn’t bad for a bullet proof sync solution with version history, imo. I also have my vault backed up on each client locally for extra protection.

I’d love to suggest Logseq because FOSS, but man does the android app suck.

That said, I find Obsidian really lacks in the simple to-do/checklist function. So I use Quillpad synced to my Nextcloud server for Google Keep-like functionality. Everything else goes into Obsidian.

I have used Nebo as well and instead of exporting I did a select all, copy and paste. Not very elegant but it did work to sort of “convert” to markdown.

And the file names are not the note titles like Obsidian (and logseq I believe)

I’m using Quillpad

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.