I've known you "since after" I met you. This looks so absurd!
Solution 1:
I can understand why someone might say "I've worked here since just after I graduated", on the grounds that they didn't literally start work the moment they were handed their certificate - although "since I graduated" would be perfectly acceptable.
I suppose it could be argued in a similar way that the two people didn't really 'get to know' one another at their first meeting, but even so "I've known you since after I met you" sounds pretty absurd to me.