Field change in connected table triggers field change in local table's corresponding record - possible?

Actually there is.  Almost nobody knows this.  You can enter the API call to change the key field of your table to a formula field.  It is not really documented and to be truthful, I have not used this myself.  I heard about this at EMPOWER and have yet to have to opportunity to et answers from QuickBase as to how safe it is to use.  Perhaps Quick Base support can tell you.

The API call is here
https://help.quickbase.com/api-guide/index.html#setkeyfield.html

So you would go to the table in question and type this after temporarily disabling the need for Application Tokens.

I would suggest testing on a copy of your app first, but that could be difficult since you have a connected table.

https://target_domain/db/target_dbid?a=API_SetKeyField&fid=7