How to use Flow (or other) to update inventory of variants in Bigcommerce?

In Flow I am trying to send Shopify inventory changes to Bigcommerce. I have the other direction sorted out. The Shopify trigger webhook is working. Regarding the next step, I am able to update the inventory of a Bigcommerce product that doesn't have variants, by entering the product ID in the Bigcommerce App under Update Product, like this...

The new inventory is entered in a field further down.

Unfortunately, when I enter an ID of a variant then it says "Not Found" when I run the App.

Do I need to create a Custom Function and how would I do that to adjust the inventory of variants? Does someone have the time to explain or write a script? Should I use some other product?

I know there is Zoho Inventory which can connect inventory between shopify and bigcommerce, but I plan on continuing to use shopify to store my order info and be my inventory "source of truth". Not sure if I could use some 'aspect' of Zoho Inventory.

Already really late here so I'll check for replies in the morn.

Thanks