Design of Spring boot endpoint that returns large amount of data continously until complete

Question

I'm developing a web application using Spring Boot and React, that needs to return information about hotels from different cities:

• As an input of the controller, I receive a list of cities that can go up to 50. Additionally, the user can filter the search using many criteria (hotel rate, price, etc.)
• As an output, I have to return, for each city, the average cost of the hotels in the town plus a few other pieces of information.

I will be using third-party API to fetch hotel data (like Booking API, for example).

The problem is that fetching hotel data for a single city can take some time (up to 5 seconds). So bringing data for 50 cities can be very long! I could parallelize API calls, but depending on the number of users using the application, it would still take too much time to display the 50 hotel results.

I don't want a user to wait too long before displaying the hotels' data. As soon as information is retrieved from a single hotel, it's returned to the front-end. This way, UI, hotel data are continuously displayed until all are processed.

The same behavior you can find when you're searching for flights and flights are consistently displayed.

I was thinking about the following solutions:

1. Use Server-Sent Events on the back-end. The problem is I do not know how to tell the front about all data returned, and complete the process is complete.
2. Use a "classic" REST endpoint, and front-end to fetch parameter continuously until it receives all the data. On the first call, frontend will be collecting data, for example, of 5 hotels, then 7, etc.. I will be using different HTTP status codes to notify that all data is processed or not.

Could you tell me what would be the best way to do what I want? The server-sent events look good to me but seem to be more suitable in case of notifications.