Default signature per email address not email account
Based on several hours of research and digging around in .plist and other files, I feel confident that there is NOT a way to do what you (and I) are looking for in the Apple Mail macOS app. Sorry.

Here's another answer that helps confirm this.

This would be a handy feature that couldn't possibly be very hard for Apple to implement based on how they have already designed a very flexible UI for assigning signatures to accounts, and the structure of the underlying .plist files seems conducive to the concept, too, if the aliases just had account-like IDs. Too bad.