Yes, your diagram is what I meant by that option.
Unfortunately, you seem to have conflicting requirements. You cannot have an isolated system with the two Pis and connect to one of them from outside of that system. Just can't be done.
Its worth going back and looking at what you're trying to achieve. A few questions
1. why do you want the separate file server, what is its purpose?
2. why not connect the SSD directly to RP2 which WILL give you the maximum speed connection?
3. what speed is actually needed to stream 4k60Hz iso files (someone else might be able to answer this because I can't)?