The exact opposite of TikTok is reading a book

Actually it's KotKit

Hm, ok. Lets say that the goal of TikTok is to be overstimulating, to catch your attention and provide bursts of dopamine. I'd argue that any form of entertainment (like books for example) aim to do the same, albeit on a less intense scale. Yes, you could say that books require patience and commitment unlike the short form content of Tiktok. But I wouldn't say they're opposites.

Probably watching paint dry would be the opposite? That would require an almost meditative/zen state

Ona scale of units consumed, yes. A TikTok lasts seconds whereas a book takes days to compete.

However, I've read books more engaging than some tiktok videos.

Idk, maybe Solitary Confinement