For progress route luke I came up with the idea of creating a situation where Luke would be jealous of Sandro because you described Luke as wanting to be Sandro's only person (and I hope Sandro doesn't betray Luke's warmth).
Obviously, someone as warm as Luke won't do anything strange that goes against his personality with a short temper and maybe not even having sex scenes. Might make Luke attack Sandro with enthusiasm and have a little nsfw scene happen. But decorating the story to make it flow smoothly is the job of an expert like you. If you are interested in my guide to Luke route
P. S. I still want to see his NSFW scenes hahaha.