I use both Resend and Sendgrid and I have used almost all other providers in the past.

The part where Resend stands out is the time to go live and ease of use. All other prodivers require you to verify your startup and they vet you and decide if they want to offer you their service or not. This creates a barrier for people with side projects and really new startups. Meanwhile with Resend you can start sending mails from day 0 once you verify your domain. They also use AWS underneath and don't have their own infra like Sendgrid or Twilio.

Resend also let's you easily use smtp with same api key as password. It's a small feature but it has come handy many times trying to configure email notifications on metabase and coolify.

I remember once I needed transactional email service for an experimental project and both AWS and sendgrid didn't want to provide me service. They actively try to vet projects so that spammers don't use their infra and bring down their deliverability and reputation.

Now for your use case since sending a lot and batch emails is going to be a core task, amazon AWS SES is the best choice for your. Most likely you will face issues where your users are uploading email ids without users' consent and when you email then they report it as spam. AWS will block your SES access in this most likely scenario. Spammers taking advantage of your system to delivery spam emails. You can study how other platforms handle this and add protections to discourage this behavior.