For what it’s worth, I use the Notes app in Nextcloud, and I like it. It does have some limitations, as Max says, but it does support Markdown relatively well, and it does what I need to have it do.

I’ve had considerable success otherwise in using the Reminders app on my Mac, synced to Nextcloud, for some of the things I formerly used Evernote for: when I have a new bill to pay, for example, I have an Applescript set up that creates a new Reminder, with a link to the PDF of the bill.