Luxray's Gen8 moveset is so weird to be honest..
I mean what sticks out is it ThunderFang being REMOVED from the Level-up moveset and only getting Crunch at level 56
Wild Charge at LEVEL 80.
Meaning that you'll have a Bite + Spark Luxray
for most of the playthrough.
Also, Psychic Fangs, Play Rough and stuff like Superpower and Iron Tail are TR's. It's currently unclear if those make it to BDSP.
But there's always the good chance they'll do a BDSP-adjusted movepool/learnset.
Just to add to @Ignition
's post. He talked about Facade, but Guts is a hidden ability and I don't know how accessable that's going to be in BDSP.
As for the 4th moveslot... Just look at what type your team is most weak against.
You have leftover Manaphy? Like legitimately obtained Manaphy?
The only reason I had leftover Manaphy because I had an R4 on which I played the Ranger Game - which I legitimately transfered to my not so legitimate Platinum at the time (I got a legitimate one later) and would constantly hatch, transfer to White and re-upload the save file that still had the egg (and repeat). But that's as close to hacking that I've ever done honestly. But with the Switch and Home you can honestly do the same thing, if you just create infinite Nintendo Accounts (start up game, transfer Manaphy to Home -> Transfer to main account -> Restart game on secondary account, etc.