The primary challenge with building local first software is the sync layer. The current 3rd party offerings are not mature. And people have been working on these for a few years. Electric SQL comes to mind.
I'm curious on what you experienced that caused you to come to the conclusion that 3rd party sync solutions are not mature. There are several 3rd party vendors like Ditto that have been building local first sync solutions that are used by very large companies with success.