This is done via an automation, right? If I were you, I would create a link between the Task List table to the Main table, and modify the automation to create a link between the two records
The automation you’re using to update the single select field in Main can now refer to that link.
Here’s a working version. To view the setup (formulas, automations etc), duplicate the base by clicking the title of the base at the top of the screen, then the three horizontal dots on the right, and then the “Duplicate Base” button.