I think Quillpad would be worth taking a look at. I needed indentable checklists more than syncing so I switched to NotallyX, so I don't remember if it has all of these things, but it seems like a good fit

Edit https://f-droid.org/packages/io.github.quillpad/

With Quillpad, you can:

• Take notes with Markdown support
• Make task lists
• Pin your favorite notes to the top
• Hide notes you do not want others to see
• Set reminders for events you do not want to miss
• Add voice recordings and other file attachments
• Group related notes in notebooks
• Add tags to notes
• Archive notes you want out of your way
• Search through notes
• Sync with Nextcloud (requires the Nextcloud Notes app installed on the Nextcloud server used for syncing)
• Backup your notes to a zip file which you can restore at a later time
• Toggle between Light and Dark mode
• Choose between multiple color schemes

The github (https://github.com/quillpad/quillpad) leads me to believe there may already be a desktop app. Nextcloud syncing looks to currently be experimental

Just use the Nextcloud notes app? It does markdown, technically it only edits md files in the notes folder. But markdown means lists

Edit :but alarms seems to be a no, you could use task.org if you got a Nextcloud anyway, checkboxes seem to not work as '[ ]' but should

Edit 2: apparently I was too stupid to type properly, it does have checkboxes

Desktop: https://imgur.com/a/uBRibEG Mobile: https://imgur.com/a/fuljOtB

Not so sure about best but it does work kinda like you want to

Obsidian