Bulbapedia says that Rock Polish can only be learnt by transferring from other generation, as you already know, Rock Polish is no longer a TM/TR.
In Gen 8, Golurk can learn Rock Polish only when obtained from:
Gen 7 or earlier (move is level-up/tutor/TM/HM in Gen 7)
Source: /learn Golurk, rock Polish in Showdown!.
Unfortunately, it's not possible to get it unless you transfer it from the older games.
The only way you could boost it Spe apart from Rock Polish would be something like Choice Scarf. Or just like how DrowsyDrakloak said, you could also run it with Trick Room support (it's fairly slow at 55 base Spe, which is generally enough).
Hope this helps.