Great choice! I see the Spigen website is offering a 18% promo discount for Tax Day here in the US which is a good deal.
Spigen always delivers quality products. The Silver Arctic looks nice for sure and should still maintain the slim profile of the S8+ for you. I've always thought the Otterbox models to be a bit bulkier and add too much grasp factor to the phones. You should have ordered their Non-glass screen protector also. I've always used their screen protectors from day one on all my phones. I ordered two packs of the non-glass type.
It's always good to have accessories in hand for arrival to dress up the new puppy.