Ask HN: How to encourage code review?

I work in a small development team where each developer is considered a specialist: we develop and maintain code based on our field of expertise, in the programming language of our preference. However, the resulting bus factor is usually exactly one. It is currently not desirable to enforce code review as a formal part of the process because the company is still in a phase of product validation and time to market is the primary concern. However, software quality is growing in importance.

How do I gradually implement a culture of collaboration through code review without hurting time to market and agility too much?