Obsidian is not FOSS, but you can switch to it for now because the whole idea is that it's just a folder of markdown files. I recommend shopping around by pointing each app at the same markdown folder, so you can see your same notes without having to worry about complex migration. Being able to look at all your notes gives you a better idea of what will suit you.

Also, I recommend Pandoc for translating between document formats. It's not not absolutely perfect, but it is wildly good at dealing with the complex problem of translating.

The simplest thing you can use, IMO, is Marktext. It's basically Notepad for markdown -- no file manager, no special features on top of the markdown syntax, etc. Beyond that you start getting into what features you want on top, at which point you really do just have to test them out for your use case.

As far as options go, you have basically two options as far as systems go:
- A built-in sync in the program between the desktop and phone version of the same app (i.e. obsidian, Joplin)
- Use a tool like syncthing for sync between devices, which allows you to use any app you want for actual note-taking, and allows you to use different apps on phone vs. desktop.
I do the latter, and use Zettlr on desktop (more document than note-centric) and Markor on android. The issue with the former is that a great desktop app might not have the best phone version. Also, the apps that do sync typically use an internal database that you can export as a folder of markdown files (i.e. Joplin), and don't actually just look at a folder of files. This makes testing out different editors kinda hard, unfortunately.

The other wierd variable is that some apps are literally just a WYSIWYG markdown editor (Marktext, etc.), whereas most of them are markdown editors with Other Stuff On Top™ (Obsidian, Zettlr, LogSeq, etc.). Not all apps implement the same flavor of markdown (which can be maddening, but you can use pandoc to change markdown flavor), but if you rely on a specific app's special flavor of garnish on top of markdown, it becomes harder to switch to another app in the future if you prefer its functionality or UI. Just something to keep in mind.

For me personally, one of the make or break traits is a good table creator. Making tables by hand in Markdown is a maddening, so having a GUI way to do it makes a huge difference if you end up needing to make a lot of tables. That is really hard to find because it is hard to automate Markdown table formatting in a foolproof way. As far as I know, the table plugin in Obsidian is the best way to do that by far at the moment. The Zettlr devs are working hard on rewriting theirs from scratch to be way more robust, but that is WIP.

tl;dr Just pick a Markdown editor, and you can shop from there as long as they store their files in a simple folder.

Since you’re already using Standard Notes - have you checked out Awesome Standard Notes? You can use the community extensions - editors, themes, etc. - even with the free plan.

It’s my main note-taking app, but I also got the 5 year paid plan for $150 (IIRC) a few years ago, and prices have increased substantially since then. If I weren’t locked into a lower rate, I’m not sure I’d subscribe at the current rates (though I would look into the self-hosted Pro discount before ruling it out). That said, if you don’t need note linking, queries, and those sorts of things, then I think the free plan of Standard Notes + community extensions is a great option. If I self-hosted the server, the main thing I’d be missing over the paid plan is nested tags.

Logseq (repo) might meet your needs if you’re okay setting up a sync service like Syncthing on every client you use. Of course, you could use Dropbox, Google Drive, etc., but I recommend against it without a layer like Cryptomator in between, since your data is store in the clear. IMO it doesn’t really make sense to self-host Logseq - just use the native app that’s available on basically every platform. I find Logseq kinda confusing, honestly, but it has a lot of compelling features.

SilverBullet may be what you’re looking for. It must be self-hosted and has a PWA instead of native apps, but the PWA on mobile at least is quite good. Since it uses Markdown files for its notes, you could use it with some other tool on the machine hosting those files, if you wanted. I have it self hosted myself and it’s the best alternative I’ve found to Notion and Obsidian when it comes to querying my own notes and so on.

Someone else posted about Outline and I think it's a fantastic, polished option. I know that you said this is for solo use, so you probably don’t care about its collaboration features, but you also mentioned managing personal projects, and its integrations (e.g., Airtable) could be useful for that. I have it self-hosted and it is a bit more complicated than other options, but I don’t think I ran into any particular issues. I’m using it with Authelia as an OIDC provider and can share my docker-compose file and other config if that would be helpful. They also have a paid, hosted option, which you can try out for 30 days if you want to see if it’s right for you before you put the time and effort into self-hosting it. One of my most-used editors in Standard Notes is the Rich Markdown Editor, which is based off the editor used in Outline. However, unlike SilverBullet and Standard Notes,

Hedgedoc is another option that may be worth looking into. It’s my go-to collaborative editor / gist replacement. Personally, I prefer it over Outline. Its main shortcomings are that:

• it must be self-hosted (though you could use HackMD aka CodiMD, which it was forked from, as that does have a hosted version)
• it doesn’t have an app (on any platform - not even a PWA)
• it doesn’t have any sort of querying capabilities, and
• it doesn’t have any sort of Kanban-like tool.

But it does have several built-in integrations, like Mermaid and multiple other diagramming tools, inline images (just drag and drop), syntax highlighting for code, Gist embeds, Youtube embeds, optional Vim/Emacs keybindings, a slide deck presentation mode, inline CSV tables, etc., and that’s all without needing to mess with plugins or switch between editors.

I hadn’t used AnyType before today, but it’s been on my radar since late 2020, and it’s pretty powerful. It’s not perfect, but it seems to check off everything you’re looking for. It does have a bit of a learning curve, but it’s been easy to jump in and take notes.

It’s hard to know which to recommend you try, though, because your list of criteria don’t all map neatly to features. For example, what do you want from planning vs managing personal projects? What do you mean by “journaling?” Is having a “journal” section where notes get dates sufficient? Do you like the way Standard Notes or Logseq handle journaling, or are you looking for features like what jtxBoard has?

I’m assuming the following for my table below:

• Quick Notes - easy to create a new note and just write some stuff. Needing to fill out any required fields (even “title”) make this a ❌
• To-do lists - checkbox lists. You have to be able to add a new item by pressing enter and mark an item off just by checking the box.
• Managing and planning projects:
  
  • Kanban / Trello style board - without needing to integrate with a non-FOSS third party service (this is why Outline gets a ❌)
  • Linking to another note in the body of a note (Standard Notes lets you create a link in the tag bar - this doesn’t count)
  • Embedded querying of your other notes, treating notes like objects - really the thing that makes Notion so powerful
  • Easy table editor
  • Diagrams - Mermaid, Excalidraw, or a similar plugin that works natively

  
  

• Easy to use - auto-saving of notes, automatic synchronization that “just works,” rich text copy-paste, etc..
• Offline mode - You didn’t mention this, but I’m calling it out since it’s otherwise easy to take for granted.
• Publishing - you mentioned not caring about collaboration, but being able to publish a note is still useful in solo-only workflows, as it gives you a way to reference it directly from a bookmark, some other tool, etc., potentially from a device where you aren’t authenticated.

[1]: For Standard Notes, I’m not assuming that you’re self-hosting the server, but I am assuming that you’re installing community extensions, particularly Rich Markdown Editor or something similar.
[2]: For Silver Bullet, I’m assuming that you’re installing community plugins.

I recommend you try AnyType and/or SilverBullet first, depending on which one looks more appealing to you.