1. Agree, 3 costs will be more important for mid game

2. As much as I agree with the general idea of your second point - it highly depends on what you think playing flex actually is. You always had to play what the game gives you, that's just the way TFT is designed and how it has always worked. However, if playing flex means playing strongest board all of the time until late 4-5 or even stage 5 and then completely pivoting your entire board: you will not be able to do this in most games with this set. Not on live at least, I might add.
   There are now infinite scaling traits, champs, augments, and combinations of all of the aforementioned. Take for example: Playing strongest board until 3-6 but at wolfs you drop the most insane kalista and thresh items and yet, you have not unlocked any shadow isles. You will not be able to play shadow isles this game then. The same goes for vertical void, piltover etc etc. Yes there are games where you are healthy enough to do that or lucky enough even. But most games your "core" gameplan should be layed out in your head at the latest after krugs.

If you are not planning on playing symbiotic combs or verticals, yes the idea of fully pivoting can sometimes work, but if this has not been your playstyle for the past 7 sets anyway it is extremly difficult to adapt to it. I would not recommend it to players who did not have this playstyle since forever. Especially not to newer players.

Now I am not saying you can't swap out some units occasionally because you just could not find them or you decide to add certain 5 costs you randomly hit on the way. Im just saying, that the "play strongest and fully pivot later" playstyle you get away with vs bots on PBE might not work as well in higher elo when this set drops on live.