[ArturT]

If you have no tests at all I would start with simple E2E tests to ensure your critical parts of app are working like sign up page and billing. You can add later other important sections of your app to ensure happy paths for your features are working fine.

When I started my own product for Ruby & JS developers https://knapsackpro.com I was doing mostly unit testing and later relayed on E2E tests for testing user dashboard to ensure happy paths are covered.

It's very easy to introduce weird bugs and it's much faster to run CI build to test your app than manually verify if it works correctly in your browser when you make some changes.