FOSS doesn’t mean free as in beer. Even so, you can get support for all of the things you mentioned, and more, for free. You can find the link from my comment above by searching for “Awesome Standard Notes,” but since you mentioned editors specifically, this will take you directly where you want to go: https://github.com/jonhadfield/awesome-standard-notes#editors

Edit to add: you can enable a community created editor (other than the ones that are included in the paid subscription) by going to Preferences > Plugins > scroll all the way to the bottom > and then, in the text box under “Install Custom Plugin,” paste the url (like https://mortalhappiness.github.io/sn-tui.editor/ext.json for the Tui Markdown Editor), then click Install, review the extension info, and then click Install again. You can follow these same steps to install a different theme, too.

There are several Markdown compatible editors in that list, so you’re free to choose the one that works best for you.

The Append Editor supports multiple types of Markdown, including Github Flavored Markdown, as well as LATEX. You can get an inline check mark by prepending a line with - [ ], but when you press enter it doesn’t make the next line a checkmark, nor can you click a checkmark to check/uncheck it, so it’s not a good choice if checklists are your main use-case. Some better options:

Tui Markdown Editor requires you to manually start a checklist the same way, but then a new line in either markdown or wysiwyg mode will also have a checkmark. You can also check/uncheck in wysiwyg mode with a click.

The Rich Markdown Editor is also good for checklists. It only offers a wysiwyg experience where you type in the same view as the final one (rather than having a split view), but there’s no top menu. You can still type the normal Markdown characters with exceptions when it’s something that requires multiple characters. In that case, type / on a new line to get a list of options, then type a couple characters and choose the best option. For example, I can’t just type - [ ] and get a checklist because it escapes the characters so that I get a list that would render the [ ]. But I can type / and then type “checklist” or “todo” (really just 2 characters of either of those words) and choose that option to start an in-line checklist. And if I hit enter, the next line defaults to being another checklist item. You can also click the checkmark to check or uncheck it.

For a dedicated task list, Scratch also works well. It’s not Markdown behind the scenes, though (but it is still readable plain text). You can start a checklist by clicking the checkmark icon in the top menu. Like the Rich Markdown Editor, a new line will create a new checklist item and you can click the checkmark to check/uncheck it.

You also might check out the “Advanced Checklist.” It’s in the “Official Extensions” link but I just tested and was able to install it on a free account. (Note that you can also self-host the “paid” editors other than the Super editor and use them for free, though that does require a bit of extra work.)

There are other editors on that page and also other community written editors in general out there. The Marp Editor lets you create presentation slides. The Mermaid Editor lets you create diagrams. Heck there’s even a Music Editor (for tablature, not for directly generating audio). There are a couple sketch / whiteboard apps. I’m not saying it has everything you could ever dream of, but there’s a lot and it’s easy to extend for anyone with front end web development experience.

Regarding your point about Simplenote syncing faster: I assume this is because Standard Notes is end to end encrypted, but Simplenote is not (Unfortunately generally e2ee adds additional overhead to syncing operations as the server cannot optimize nearly as much). There may be more to it than that, though. I’ve looked at Standard Notes’s code a bit but haven’t looked at the server code for Simplenote - as far as I can tell, Simperium, the backend it uses, is not open source. (The frontend is, though.)

That said, I’ve never noticed Standard Notes taking particularly long to sync. I don’t have thousands of notes but I do have several hundred (pretty close to a thousand). Were you seeing bad performance with Standard Notes or just worse (but still fine) performance? And if the latter, can you share some more details?

For me, not being end to end encrypted is a dealbreaker for my private notes (unless I’m self-hosting the app, that is). I’m not comfortable with someone at the hosting company just being able to access my notes. But that’s not important to everyone, and if it’s not important to you and if Simplenote otherwise does everything you want, then by all means use it - it’s a great choice.