Radar vs Socket.IO

We are starting to work on a web-based platform aiming to connect artists (clients) and professional freelancers (service providers). In-app, timeline-based, real-time communication between users (& storing it), file transfers, and push notifications are essential core features. We are considering using Node.js, ExpressJS, React, MongoDB stack with Socket.IO & Apollo, or maybe using Real-Time Database and functionalities of Firebase.

We (my team) are building an App where we want to have Bi-directional texting, Single Directional Picture, and audio transfer.

We are building all this using Flutter.

There will essentially be 3 apps, 2 Mobile-based (Android and iOS) and 1 Microsoft Based. We've built up most of the code already, and made a few major mistakes but fixed it(namely had no proper state management).

How things will work:

Person A has a Mobile app 1, Person A presses a button that sends a "communication request" into a Pool of requests. Person B on Desktop App chooses a "communication request" from the pool, and engages in Bi-directional texting with Person A. Person B also opens communication with Person C who is on Mobile app 2, and they engage in Bi-directional texting. Person C will be notified of communication requests through Push Notifications.

So far we've been using Socket.IO, however, I'm starting to think that's not the best.

A problem we've encountered so far is that Person A(Mobile App 1 User), is the person who sends a "communication request" into the "Communication Pool". The Mobile App 1 User, can "cancel" the communication at any point in time. When they do that, I would like for a notification to be sent to Person B, the Desktop User, For them to pick up another communication request.

I am not sure how this should be done however, should it be done in the Back-end, then how does the Front-end get notified of the change?

Any advice on which to choose?

It really depends on how you want to do it. If you use Socket.IO, you do have to have your own server running somewhere else, a good free one is Heroku. Pusher is nice if you know what you are trying to do, know your limits, and how to do it. While Socket.IO allows more flexibility if you need to do something crazy, Pusher is great if you're just doing standard real-time data transfer because there is minimal setup. The only other thing about Pusher is that it is not entirely free, so if you know the pricing, know how much you are going to use, and are willing, you could easily use Pusher.