Hello! Here is a mini-guide on how to get Endings 6 and 9 for Lumine:
Ending 6 – Choose to work at the ice cream kiosk. Assemble the orders correctly – when an order is done right, the heart decoration area will start flashing. You need to manually add a secret heart to at least 5 different orders! Also, do NOT flirt with Tartaglia or the customers so you can unlock the Picnic route. During her confession, a new option will appear: "Business proposal!".
Ending 9 – Go to the kiosk and flirt heavily with Tartaglia and the customers (to head towards the Hotel route). While assembling the ice cream, click on the ellipsis icon (...) 1 to 3 times to trigger anxious thoughts. Once you are at the hotel, choose the option "Stop it!". Lumine will realize she is losing you.
Hope this helps! Good luck!