Weather routing is already prevalent over existing satphone connections. The GFS model that is typically used for this purpose has low enough resolution that there's marginal returns to additional bandwidth or reduced latency.

You can interpolate the GFS very accurately in terms of wind over land and close to land with WRF-ARW. There is highish resolution land use data and good elevation models. This is something you can do on your laptop even these days.

However I have no idea of good it is at wave and currents.

You can also put the compute and the data on land, and do routing as a service. The decision bandwidth (i.e., ideal heading/route) is miniscule.

You can also get real time data via ham radio although it’s extremely slow. It still works for navigation adjustments and optimizations. I fail to see how this is relevant beyond crew morale.

Well, routing seems like something that needs just ship position. You can easily send that over, for example, iridium network and as long as you won't try doing that 10 times per second (because why?) if savings were actually that big, it could already be done, even many years ago