Hi Matthew,
We looked into this in some detail.
We recreated your product variants and tried using Shopify Assistant to sync inventory on purchasing a variant and also found it very fiddly.
However, we did find an app called SimpleSync[1] that could do the job. It sync inventory across products/variants sharing the same SKU. Does that work for you?
If you require syncing of variants of different SKUs, we're happy to look into developing a custom solution for you.
Zac