Typinator is using this regex for auto-capitalization:
If we tweak this a bit we can use it in KM as a Typed String trigger and get close to what you want:
The drawback is that any "at" will not get auto-capitalized.
We could exclude the pattern at followed by a number but then we would be in trouble with things like "i am" (would become "I Am"), or with things like a preceding "1." or "U.S." which should not trigger the capitalization.
(Probably the macro could be optimized by running the trigger through a second regex instead of the simple filter.)
So, this is far from being perfect, but maybe it's good enough. Just remember to manually capitalize any "at" not followed by a number
Note: Of course, you have to disable macOS' auto-capitalization feature as shown above.
[test] Auto-Capitalize (Except "at").kmmacros (2.1 KB)