hmm if you're that early, then go with Roselia for now, Onix has pretty low Attack and won't be of much help, while Roselia is decently strong. Lapis ward is accessible after your 3rd gym battle, so it's pretty soon. You can get Roserade between badge 4 and 5, also earlier than Steelix which is 6-7. And yes, let it hold the Metal coat while using the Link stone to evolve it.
Also Tynamo is really weak and useless at first, i suggest leaving it in the daycare until you can evolve it.
If you want more detailed guides there's several on this forum which are always up to date. Ep18 Item guide:
https://www.rebornevo.com/forums/topic/41661-item-guide-v18-void-kissed/#comment-807677
Pokemon location guide:
https://www.rebornevo.com/forums/topic/42836-void-kissed-pokemon-location-guide/
https://www.rebornevo.com/forums/topic/43367-e18-pokemon-locations-spreadsheet-all-wild-and-event-pokemon-plus-more/
Walkthroughs (Video and text format, also sidequest list)
https://www.rebornevo.com/forums/topic/48337-v18-full-walkthrough-guide/