Make sure you go to your User's Library (that's what "~" indicates) and not the Library associated with your Mac HD. It can be found in the User's Library, never fear.
For me too the Finder search does not reveal the .plist for Mail. That could be a "safety" measure by Apple to protect us from ourselves.
On a Mac since 1984. Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
