help needed to switch from Avalon MM to Avalon ST interface

Audio and video formats can benefit from a stream interface. For metadata it depends on a few parameters. I don't know about the standards that you are using, but if you can transport the metadata in packets, you could use them in a stream too.

I wouldn't advise modifying third party IPs though. It will make it harder to upgrade, and get support from the supplier. Another way of avoiding latency and arbitration is to separate the avalon MM busses. You don't need to connect all the masters to all the slaves, so if you are able to only connect the slaves to the masters they are supposed to talk to, it will reduce the arbitration.

If you are implementing yourself the IP that does all the transfers, you could design it with multiple MM masters, and connect each one of them to only one slave, and have that slave only connected to your master. With 1-1 connections you would reduce the latency to the minimum and get rid of the arbitration logic.