How can I find the date a device was added to Apple Account?

It’s not exactly easy, but at least for “adds” …

… if you possess the tools, skills, and are willing expend some effort manipulating csv data files into a format usable by you …

… those dates can be extracted from your own account data available for downloading from:

https://privacy.apple.com/

Unless you see it right next to the device information (I don't see it there) then I don't think it is anywhere else. Once a device has been removed there is no trace it was ever on your account.