Given the requirements of Curiosity I believe the current approach you are taking is reasonable - whilst the snap could work as a classic snap, it would appear that the use of desktop-launch and perhaps system-files to allow the snap to enumerate all applications should allow the snap to work under strict confinement, providing a more secure solution for users.
However, note the desktop-launch interface is super-privileged and so requires publisher vetting (similar to the process for classic confinement too).
As such, +1 from me for the use-of and auto-connect of desktop-launch for curiosity. Regarding system-files - to see the actual applications installed on the host you will need to look under /var/lib/snapd/hostfs/usr/share/applications instead. So please change your system-files slot to specify this path, and this should then be named: hostfs-usr-share-applications so that users can understand it’s purpose better. Finally for personal-files - the home interface should already provide the access which you need.
As such, +1 for use-of and auto-connect of system-files named hostfs-usr-share-applications for read access to /var/lib/snapd/hostfs/usr/share/applications as well.
-1 for personal-files access to read of $HOME as this should be available via the home interface. Note this does not provide access to say ~/.local/share/applications (which I wonder is what you really need) - in that case you could specify a personal-files instance with read to this path $HOME/.local/share/applications called dot-local-share-applications - in that case, +1 to that from me.