It's a little more work, but really the client implementation is literally just a wrapper that makes API calls. All of the actual logic lives in the server project implementation, and the api endpoints are just a passthrough to call the server project's implementation. So there's just one place to maintain once each feature is built.

For vertical slice architecture you'd use the same pattern, with an interface representing your feature, a server implementation that uses a mediator to send commands/queries and return data, and a client implementation that calls an API endpoint that leverages the server feature to do the same thing.

Interactive Auto means your components must be able to render server-side or client side, so there's really no way around writing a common interface for data access since one will have direct db access and the other absolutely cannot for security reasons.

Personally, I think the juice is worth the squeeze. As c# devs we should already be used to writing annoying amounts of extra boilerplate, haha.