Without really thinking it through, I see a various technical problems with that.
But, if you're really into instantaneous transfers between shares and using SMB, then why not experiment with
Don't limit shares to specific drives. I've never particularly understood why people actually do it except for OCD reasons
Set up a root share for unRaid ( @SpaceInvaderOne has a video that covers that).
All moves now between shares are instant, without the cache drive involved at all
The first point is technically optional for this, but would alleviate the concerns as to why the files aren't actually confined onto the disks specified.