Since v.2.23.0 of Automatic Related Products extension, we have added an option to  use only the deepest level subcategories for the Same As condition. In order to obtain the feature, the subscription functionality module should be installed additionally. You can find the 'amasty/module-automatic-related-products-subscription-functionality' package for installing in composer suggest. Please note that the modules are available as a part of an active subscription.

After installing the extension, the functionality will appear in the Amasty Automatic Related Products extension's configuration. Kindly enable Use Deepest Subcategories for ‘Same As’ Condition setting:

If the setting is enabled, then when the Category is the same as Current Product Categories option is chosen, the deepest subcategory or categories, if there are several at the same level will be output for the product: