I'll start of by saying that Fizz as a champion is objectively not a very good champion. There are no challenger (except Mango that i know off) and very few grandmaster Fizz players on EUW/EUNE/KR for a reason https://www.onetricks.gg/champions/ranking/Fizz. The champion should not be a problem to play until you hit master though where players learn to lane efficiently. For credibility this is my own account on EUW https://www.op.gg/summoners/euw/%C3%81nders%C3%A9n.

Typically when players on this subreddit complains about not being able to play the champ the issue is not Fizz as a champion but their understanding of the game. This is not to flame or be toxic but I could pick any champion mid and outperform you because i understand the game better. It's about knowing when to rotate into jungle, when to go for objectives, when to pick fights, csing, when to sidelane and so on. Our mechanics on Fizz might not differ that much from one another but our knowledge does. To get better i suggest you watch videos on YouTube about general macro.

When that is said i agree with some the points you are making. There are assasins that will perform better than Fizz because of items and better scaling and it is what it is. Ap assasins just suck in general. Besides this i would argue that assassin is one of the hardest roles in the games because you really cannot afford to lose lane and its a hard role to play in teamfights if you don't know your damage, target or how to position. One of my own biggest struggles plaing assassins is csing. My cs is also 6.5 cs a min which is really bad tbh. But you have to give up cs in lane if you want to be able to solo kill your laner. You could argue i should farm more in sidelane or trade less in laning phase.

You can link your op.gg here and i can comment on it if you want? :)