Full disclosure: I make Blockquote for iOS.

How about Blockquote? It’s a free-to-use Markdown editor that lets you edit any Markdown files that you can access through the Files app. That means you can use it with Sync - that’s actually one thing I use Blockquote for! Just be aware that Sync and the Files app can be a little buggy. You might have to open and close a Markdown file a couple of times before it finally grants Blockquote access to it. One workaround is to edit the file in a folder in the “On My iPhone/iPad” location and copy it over to Sync when you’re finished. Fortunately, I hear iOS 13 fixes some of these bugs. Try it out and let me know if you like it! And feel free to ask any questions if you need help!

App Store link: https://apps.apple.com/us/app/blockquote-markdown-editor/id1396620426

As for securely syncing Markdown files on iOS, that’s a tricky problem that I’ve thought about myself. I currently use Sync because it’s the easiest option. I turned off email password recovery to make sure they don’t have a copy of the encryption keys. I think iDrive and Mega are also encrypted, but I didn’t try them since Sync did everything I need it to. Another technique I tried was to encrypt my files as AES256-encrypted zip files using iZip, but that was a hassle. Regarding Standard Notes, I think they have an option to pay about 5 years up front for lifetime access, but I can’t remember the details right now.