One trick is you can use /app/settings set app-store-urls= from your PC via something like npx serve/ python3 -m http.server within a directory with your /app YAML. This is what I've been doing to test things, since I can use VSCode to edit the /app, but also get validate/competion of the /app syntax, which is the picked up indirectly via app-store-urls.
I'm not sure it's polling interval, but if you just edit app-store-urls it will repull the custom app definations. Also the YAML does need be structured with an array (-\n+2 spaces, then app YAML) for the YAML to be loaded by app-store-urls.
But agree it should be editable in WinBox/WebFig. Or, the full set of options in YAML being expressable as RouterOS syntax.