Where to manage mailing lists for multiple websites

Where do y'all manage mailing lists for multiple websites?

I've tried ConvertKit, but it basically requires creating a new account for each site. As an added downside, there's no way to add a Subscription center for subscribers, or at least I haven't found such a feature.

Also have tried MailChimp, but multiple audiences there was buggy in the UI for some reason. It'd show I was editing a specific audience but saved the changes to another. Unlike ConvertKit you're also required to add your own address to be visible on some public pages, like profile update page.

Going to try CampaignMonitor now, but would appreciate any insights and ideas!