Load Balancing ADFS running on-premise & Azure

Pool members don't need to be in the same subnet, so that is not a problem.

There is no need to use Azure load balancer, as F5 will handle that.

You can use priority groups, your single on-premise server will be the high priority group, if down, the system will activate the second group that has both Azure servers.

You just need to make sure F5 knows how to talk with the Azure servers, probably a VPN between you DC to Azure, and the traffic returns via the F5. For traffic to return, you can use SNAT.

The speed of accessing the DC server will be faster than accessing the Azure servers, but I guess you know that.

In relation to the iApp, you can use without problems, as it will not care about where is the IP located. However, not sure if the iApp will give you the option to use priority groups, so you can change that after (disable strict updates in the iApp) or setup without iApp.

Some useful links for your case:

Priority group:

SNAT: