Pikachu uses both Gigavolt Havoc and 10,000,000 Volt Thunderbolt so I don't really see why that would be a problem. Z-moves aren't used very frequently anyway, so Lycanroc uses Tectonic Rage 1/2 times then I'm fine with it.
The difference between
Pikachu using "Gigavolt Havoc" & "10,000,000 Volt Thunderbolt" and
Lycanroc using different Z-Move, is that, both of
Pikachu's Z-Move originate from one single Z-Crystal, while
Lycanroc has different Z-Crystals for each move.
Meaning
Ash has to change around his Z-Crystal's a lot to use the other Z-Move in case of
Lycanroc or any other of his pokemon, and we rarely see
Ash change around his Z-Crystal's on his Z-Power Ring (90% time its "Electrium Z" which randomly also transforms into "Pikashunium Z").
Not to mention, we haven't yet seen a battle yet in which the trainer had changed his Z-Crystal during battle. All the time, we have only seen them change the Z-Crystal before either battle start, or they use the regular one they have.