Another vote against body side moldings, although I did manage to score a door ding about a month into ownership. That being said, the ding isn't nearly as noticeably unsightly as the moldings, so c'est la vie.
Can't opine on the spoiler since I have yet to see one in the flesh. I generally don't go for them on midsize sedans, though - sometimes it looks like "Dad-wished-he'd-gotten-a-WRX-but-settled-on-this."
If you want to drop some cash on your car, I think the two best upgrades would be the JDM black grill (although these might not give as much "ooomph" in looks when compared to your '18 - I drive a '17 where the grill can really help) and an STI rear sway bar.