Automating User Taxonomy Field Updates in Backdrop CMS with Gumroad and an External

I'm integrating Gumroad subscriptions into my site and need to automatically update a user's taxonomy field when a subscription is created. Here's my current workflow:

I use n8n to capture the subscriber's email from Gumroad.
The email is stored in an Airtable base.
Via a simple API, I create a node in Backdrop with the email as the title.
I tried using Rules to find the user by email and update a taxonomy field in their profile, but I couldn't get it to work.
Question: Is there a module, approach, or workflow in Backdrop CMS to update a user field (like a taxonomy term) via external automation? Any ideas to fix the Rules issue or suggest an alternative to connect my API with user data?

Suggestions for modules (e.g., Views Bulk Operations, Webform, or similar), code snippets, or other workflows would be greatly appreciated. Thanks in advance!