I guess usually using things like sentry or having a close conversation with customers.

Everyone does QA at a startup. There is no dedicated QA. If you're a developer, you're doing QA. Product, whatever your job title is, you're doing QA.

Everyone is QA at a startup. I hope y'all are using your own product regularly right? You should have a nice simple way to raise these issues, maybe a slack channel or something with a convenient way to turn a screenshot into an item in your todo/work system if you have one. Encourage product ownership.

Alternatively, if you don't really care about the product, everyone is also customer support at a startup - you should be quickly be able to solve regressions that customers raise. I hope that at least you are talking to your customers and are very responsive to them at a startup.