depends on what functionality you need, problem can be sharing cookies, iirc correctly cookies set on main domain will be shared to subdomain but not in opposite way.

Currently I have one site, but I want to add a subdomain for a second event. I want to share logins (so if you log-in on the page1, you are logged into page2 as well).

And I want to keep everything in one next app - I can't really find anything regarding routing for specific subdomains.

for routing it's easiest task, you can check headers origin or referer or x-forwarded-host, and you'll get current domain and do whatever you want with it, like render different components for different domains.
but as I am saying problem can be with auth, you'll probably need some workaround that will set cookies and maybe even localstorage for different domains, one of the ways is iframe, or cors request.

Ugh, I wanted to avoid that. Could I somehow utilize this property?
https://lucia-auth.com/basics/configuration/#csrfprotection

no, it still persist session in cookies on client side, and it will be set on current domain only

Hm, I see. Although if I put everthing on a slug like domain.com/something, it would work. Maybe I'll just go for that.

Is there something I'm missing here? I would assume I'm not the only one with this approach.

as I said if you'll login users through main domain then it should work on subdomains, at least cookies side, maybe there will be some changes in lucia if they specific domain cookies only.