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.
Performance is extra helpful for making real-time segment size calculations. We use elastic search and it's costly.
The multi-list approach has several other benefits. When manager / sender (and other) permissions get introduced, it will be straight forward to restrict users to managing certain lists. In addition, multiple lists allow subscribers to selectively subscribe / unsubscribe from lists.
Internally, the structure is simple. There is only one subscribers table and subscriber data is not duplicated anywhere. List (foreign key) relationships are in a separate table.
So in theory could you have 1 list and each subscriber has many tags, and then you can segment on those tags?
Also, do you have any public success stories beyond your own Zerodha campaigns? Have you compared delivery / bounce / etc. rates vs Sendy and other tools using the same email providers? Also how fast can you send emails out through SES?