The weirdest thing about Nebby as a Solgaleo is that Cosmog > Cosmoem > Lunala looks incredibly natural as a progression, with the same colour scheme, overall shape, and features, whereas Solgaleo is similar around the face but completely different everywhere else. Maybe I'm just biased because I played Moon first, but the evolution from Cosmog to Lunala looked so deliberately designed that I had completely assumed that Sun!Nebby would look like a little puppy or something, and was really surprised and confused when I found out that the first two stages look the same. It's not like there aren't already alternate forms for the same pokemon, either.
Anyway, this doujin is perfect, this really is the best way for them to reunite. Lillie left to become stronger, both for herself and for her mother, so it's only fitting that she should return as a league challenger. She always seemed to feel a bit helpless, when all she could do was follow other trainers around, so finally being able to face each other on equal footing like this makes seeing her off at the end of the game feel worth it.
I would have liked a scene where Nebby goes back to her, since then her journey would have truly come full circle (and the protagonist can get an alternate universe Cosmog for themselves, after all) but I can't expect everything from a single doujin.