Thoughts and info from Cassio mains

Hi all. So I like Cassio since some time, I occasionally play her and would like to hear from tha real Cassio playas! She was my most hyped mage update, I love what they did with her. I'm having one major issue with her though. Her mana is terrible, worse than before the rework. Now I'm not quite sure I understand what's going on here, I know E doesn't refund anymore % mana when killing the target but is that really how it's supposed to go? I checked gameplay videos of her to see how pros perform with her. Ironically I watched Bjerg miss a few E last hits and I think spamming mana a little more than me, and yet this guy doesn't run out of mana. What in the world am I missing? I had a lane where I farmed the entire time and my farm was way better than that of my opponent, but it was impossible to depush, I was all the time under tower and I would run out of mana after a while, poking wasn't an option at all. So what's the big secret? For now I have opted for the 1.5% missing mana regen mastery in the middle tree, it does help but I know the pros don't opt for that.

Also, I'm asking to make sure, in order to be mana efficient I try to aa E all minions. Is it possible that the E can hit before the aa? I'm asking cuz if it hits before but doesn't kill it maybe I have been wasting mana and am not aware.

Also regarding her build, I find it almost sad.....almost....because its just my opinion and I'm no pro and she is having success. But why are the pros building full cd on her? I like CD alot, but if there is any mage I don't mind not having CD on it's cassio, why do people not build rylai liandry on her even if they have 4 tanky opponents? They still opt for CD and somehow still deal alot of dmg without running out of mana. Thats what happened in one of the lcs matches.

Edit: also curious on thoughts about her laning phase from the experienced players, any specific things?