How can I get snappy to list all available snap packages?

Note that sudo isn't necessary. snap find and snap install will work just fine without it.

snap find only shows promoted and public snaps in the stable channel. By curating snaps in this way, users can expect a degree of quality when using snap find for app discovery.

If you'd like to know all the snaps that exist, try uappexplorer:

$ snap install uappexplorer-cli
$ uappexplorer-cli --type snap
┌────────────────────┬───────────────────────────┬───────────────┬──────┬──────┐
│ App                │ Description               │ Type          │ ❤    │ ★    │
├────────────────────┼───────────────────────────┼───────────────┼──────┼──────┤ 
│ test-snapd-cups-c… │ A basic snap declaring a… │ Snap          │ 0    │ 0    │
├────────────────────┼───────────────────────────┼───────────────┼──────┼──────┤ 
│ test-snapd-fuse-c… │ A basic snap declaring a… │ Snap          │ 0    │ 0    │
├────────────────────┼───────────────────────────┼───────────────┼──────┼──────┤ 
│ lcavassa-iperf     │ A TCP, UDP, and SCTP net… │ Snap          │ 0    │ 0    │
├────────────────────┼───────────────────────────┼───────────────┼──────┼──────┤ 
│ deadbeef-vs        │ The Ultimate Music Playe… │ Snap          │ 0    │ 0    │
├────────────────────┼───────────────────────────┼───────────────┼──────┼──────┤ 

1. Install SnapWeb which is a "Beautiful and functional interface for snap management"

   sudo snap install snapweb
   

2. Go to http://localhost:4200 using your browser of choice.

Enjoy.

It's not documented, but a list of snaps is kept in the file /var/cache/snapd/names (it contains all known snaps in the stable channel according to Alan Pope). You can inspect that file by hand to see what snaps are available, if you're happy with potentially missing a few that aren't available in the stable channel, and those that aren't in the default snap store.

I think you can also query the store directly by sending a request to https://api.snapcraft.io/api/v1/snaps/names, but it's not clear what the correct values for the required headers are, and it's heavily rate-limited. Further, it appears to return a strict subset of the names available in /var/cache/snapd/names anyway, so you're probably better off just looking at that file.