Sure, 40-amp would work, I wouldn't go any lower.
Yes there would be some benefit in staying with Victron, You would need to make sure the devices are all capable of taking advantage of that feature. If your battery run is pretty long, this communication can give the device better accuracy. It essentially like running sense wires. The other advantage could be not over charging your batteries and prioritizing charging. Again, the particular devices wo9uld need to have these specific features. But, in theory if you were putting out 30 amps of solar, and had a 30 amp DC to DC. If your upper charge limit was say 50 volts, it is possible you could prioritize solar, and limit the dc to dc to 20 amps.
I have never set this up, so I say theory, but in my reading of Victron products it is mentioned, I just haven't jumped into it. The other big item the Victron series could bring is a Victron Cerbo setup, if you were interested in storing data in cloud.