I don’t think from_candid is async…it is just a system function.
I was just making the(likely false assumption) that you were trying to compact the data somehow.
If it is just that you need binary, you may find some good stuff in Completed: ICDevs.org - Bounty 24 - StableBtree Mokoko - up to $10k. Also, check with @matthewhammer as he has an whole new stable region syntax ready for motoko.