Just a few brief thoughts.
Modifying systemd services
Use /storage/.config/system.d to create systemd drop-in directories or your modified service files. Additional units can be created too.
Persistent Logging
Can be enabled via LE Settings Addon's System tab. Use journalctl -b -1 to view last boot. Logs are visible until local file systems are unmounted on shutdown.
Final error messages
Do exist because neither /flash/SYSTEM nor /flash can be cleanly unmounted while still being used.
Shutdown scripts
Due to target dependencies network access is not possible in shutdown.sh. Use autostop.sh