Where to begin....
Well for a start, you can exclude Viktor, Syndra, Veigar, Velkoz, and Ryze from that list because they all have some form of multiplier/outside scaling/different damage type (Hex-core, Ball-ult multiplier, AP stacking passive, true damage ultimate, mana scaling).
Karthus, Cass, and Anivia are all picks practiced predominantly by mains, so you'd expect them to appear strong/fine on the surface, were the playrate to increase it'd end up looking more like Talon currently does.
Zyra and Brand i'll be honest confuse me because i'm not sure if they're strong bot, or weak mid, either way they are steadily becoming part of the support pool.
This Leaves Orianna, Annie, Ahri, and Leblanc.
Annie is gimmicky, always has, always will.
Ahri has been likely too strong for a while now but Riot seems to not care..? (So i concede this one)
Orianna is, dare i say, also a bit too strong? Regardless, quick simple number changes would fix (another one conceded)
And lastly, LeBlanc. I'll be honest I haven't seen her all too much, personally i think shes fine, but you seem to have a reason to put her on your list, so we can leave her on the fence.
Which means your choice of champions that could be seen as accurate generalisations of mid-mages across the board really only has 2 that carry any solid weight, which I've also agreed with, and in fact think they need nerfing. The rest either are inaccurate because they have either unique forms of damage multiplication/addition and/or typing that most mages dont, are low-play rate champions that are played by mains, or don't really go mid anymore. Or is Annie.
This means that
minimum would get the shaft because Orianna and Ahri are a bit strong, and then if we include junglers and supports
would get the shaft because Brand and Zyra are pretty rough.
A final note on the assassins; people are failing because they are stuck with the archaic mindset of "I go in and 1-shot", rather than using the tools given to them in the update to make PLAYS. They are simply trying to stat-check their opponents, and it costs them. Assassins have gone from "play faster" to "play smarter". Most just haven't caught on, and as a long time talon player myself, he's been working great for me, but I've been watching his playrate, ad it's way higher than it used to, which tells me people are picking him and thinking he functions the EXACT same as before. THAT is why Talon's win rate is so low.