Wordpress/BuddyPress/Buddyboss site wide push notifications

Hello Everyone

I need the ability to post a push notification containing text and a link site wide on a BuddyBoss site. But it also needs to work on Wordpress without Buddypress/Buddyboss.

I would like it to be a modal popup in the top centre of the browser window.

I would like the notification to show without the need for a refresh of the page (AJAX maybe).

I also need the notification to auto dismiss/remove itself after a set amount of time (configured in admin panel)

I want the ability to post a notification to ALL users or specific users based on Wordpress User ID.

The site will have a large amount of users at once (several thousands) so these notifications need to be efficient and not put a lot of load on the server. Maybe the notifications can be called in groups over a few seconds to reduce server load?

I am open to solutions other than AJAX if this makes it more efficient.

I will need an admin panel where I can post notifications as well as being able to call a notification using PHP.

Also the ability to style the notification using CSS for font, text size, text colour, link colour etc.

A test environment with Buddyboss etc can be provided if needed.

I am open to any questions please ask away.

Many thanks