Some notes as what I would say is near target user who generally enjoys the time I spend on Tiktok:
1. Tiktok the company is absolute crap, mainly for censorship of content and diverse creators (they even recently they A/B tested censoring private messages between mutuals). This is absolutely tied to China, though the political CCP part is a lot of unfounded griping probably. But to be clear, Tiktok the user culture and the company are very different.
2. Tiktok has essentially become the new Tumblr. The algorithmic approach means that once you give some signals, the content specifies a lot and it can be a great experience for people interested in more niche things. The "default" Tiktok is incredibly bad, but spaces for queer creators, the neurodivergent, political discussion, and niche interests such as urban planning, book clubs, fandoms, movies, tv shows, art, and more are thriving. That's something that many don't see unless they are in those groups because of the algorithm, so no "cursory" look at Tiktok will find that.
3. To answer your questions, I have had my doom scroll days but generally I keep to an hour or so now and generally feel pretty good after using it. Again it depends on what "side" of Tiktok you are on, but it avoids a lot of pitfalls. I haven't seen studies, but here's one data point for you. I'm 25 for reference on age.
If you don't feel the need or desire, you don't need to be on it. But I really think the best way to conceptualize it is a visual Tumblr with an automatic algorithm approach rather than a focus on manual curation.