I found that simply connecting a domain I obtained through Cloudflare to an email delivery service like Brevo wasn't enough to avoid my emails being marked as spam. However, I did find a solution. The key was to use the domain from Cloudflare as a custom domain with an email server service (in my case, Sakura Mailbox, which is similar to a service like Zoho), and set up proper email authentication (such as signing) in the sending settings of that email service.
After that, I connected the custom domain on Cloudflare to services like Brevo or Resend, and used PocketBase to send emails through those services. This approach allowed me to send emails without them being treated as spam.