Joplin, and what ultimately pushed me away from it was the portability of the data within it—I didn’t love that I wasn’t ultimately just working with a folder of Markdown

I believe you did miss something, Joplin "stores notes in Markdown format. Markdown is a simple way to format text that looks great on any device and, while it's formatted text, it still looks perfectly readable in a plain text editor." Source: https://joplinapp.org/help/apps/rich_text_editor/

You have have a bunch of options when it comes to synchronization:

You can just point it at some folder and it will store the files there and then sync it with any 3rd party solution you would like. I personally use WebDav because it's more convenient (iOS support) and it's very easy to get a Nginx instance to serve what it needs:

server {
    listen 443 ssl http2;
    server_name  xyz.example.org;
    ssl_certificate ....;
    ssl_certificate_key ...;
    root /mnt/SSD1/web/root;

   # Set your password with: WebDAV htpasswd -c /etc/nginx/.credentials-dav.list YOUR_USERNAME
    location /dav/notes {
	alias /mnt/SSD1/web/dav/notes;
        auth_basic              realm_name;
        auth_basic_user_file    /etc/nginx/.credentials-dav.list;
        dav_methods     PUT DELETE MKCOL COPY MOVE;
        dav_ext_methods PROPFIND OPTIONS;
        dav_access      user:rw;
        client_max_body_size    0;
        create_full_put_path    on;
    }

I was already using Nginx as a reverse proxy / SSL termination for FileBrowser so it was just a couple of lines to get it running a WebDAV share for Joplin.

Is FileBrowser doing any cross-device syncing at all, or is it as it appears on the surface

FileBrowser doesn't do cross-device syncing and that's the point, I don't ever want it doing it. For sync I use Syncthing, I just run both on my NAS and have them pointed at the same folder. All of my devices run Syncthing and sync their data with the NAS so this way I can have the NAS working as a central repository and everything is available through FileBrowser.