Thanks yeah remote play sounds like the best option thenI'd imagine you'd need two copies - unless they do one of those 'cross gen' (or whatever they're called) bundles. As for saves sharing between consoles - I highly doubt that will be a feature, as they're two different systems.
You could just use the PS4 to remote play the PS5 - that's what I do when I'm kicked off the OLED in the living room; at least that way you can just by it for PS5 and not worry about anything else.