So viel Spaß hatte ich seit unserem ersten Treffen nicht mehr.

The ‘nicht mehr’ part sounds so weird

It’s idiomatic, in German “nicht mehr” can be used to mean “no longer” ,“not any more”, or things of the sort. German can be deceptive in that the words are so similar to English in many instances, and their idiomatic usage often does correspond word-for-word to English expressions, but in many cases they don’t!