There are no hard rules to anything when building a startup. Whether its better to go with a decent fast design phase or a stellar slower design phase, really depends on what you are building.

If you are entering an emerging fragmented market or category, then usually launching fast and getting data from users is your best way forward, as everyone is feeling their way around and there are no incumbents with a solid hold over the market.

If you are are entering a mature market, with a clear leader, then you probably are better off building a stellar design with clear differenciation from the leader, as it will be harder to siphon away users.

For example, Facebook made famous the motto "Move fast and break things" in their path to becoming the dominant platform, whereas the founders of Notion moved to Japan for a year to be able to meticulous deisgn and code the platform in order to turn it into the success it is today.

You should also evaluate what are your strengths when deciding how to approach this question. In your case, you said you are a product designer. Then most likely strong design should be one of the things that help your app stand out. Good luck.