I don't have a retina Mac, but I have experimented with a 27-inch iMac in a store: System Preferences > Displays offers a choice of 5 different screen resolutions, but all of them except for the native resolution are IME blurry compared to an Electron app or Chrome (which allows me to change the size of all the elements in the viewport). My vision is not so bad that I don't notice the blurriness even on a retina display.
Graphical Emacs is the only app I know of that uses native Cocoa APIs that allows the user to change the size of most of the apps's elements. (It does not allow changes to the size of scroll bars and elements inside pop-up menus, the title bar and the menu bar, but those things don't bother me.)
I'm surprised you found the options in System → Displays too blurry. How long ago was this? Modern macs have such high screen resolutions that the slightly uneven scaling shouldn't be a problem. Indeed, on many new Macs the default resolution is not evenly divisible by the screen resolution.
On your non-retina Mac, you could use QuickRes to force "retina" resolutions.
And honestly, it's just as "in the cloud" as a web based app would be. All my email is synced to my computer from my various email accounts, and that sync is solid and quick enough that I never think about it. I can log into a new computer and add my accounts, and all my email will be there.