I'm curious, what made you go with a multiple lists approach instead of tagging users on a single list?

A lot of popular (but expensive) services like ConvertKit have moved to a tagging approach many many years ago.

It's so much easier to manage your list of emails when it's a single list and then you add tags to specific users like "purchased X". This way the email address only exists in 1 spot and you can segment on the tags.

The Sendy approach (and from the looks of it your app too) becomes very unwieldy with having to manage, parse and merge multiple lists on a regular basis.