We do something similar across a React and a Next JS app. A little different because they are both web, but same principles of letting the clients do their own thing and having common backend logic.
Also just started looking around your site. This is actually a quality engineering blog! We're actively implementing stylized components in a manner similar to yours.
We've been relying on manual testing so far. We're just starting to think about unit tests and integration tests. We don't know where to start. Would be cool if you could provide guidance on setting up good testing practices in the first place so that we avoid flaky tests all together.
If you’ve ever wanted to give your customers custom domains, send email on their behalf or manage their DNS records, would love feedback on dnspassport.com. Built this out for my current company and thinking of spinning this infrastructure out.