Right now my Cleric have 10 different Level 2 spells ... i presume it would be much more in max level. :-/
Not quite, but it will get unwieldly doing it that way regardless.
Your cleric always knows every spell they can possibly cast, but they'll only have a limited selection prepared each day - that number will increase as you level; at level 20 it will be ~36 spells prepared, including domain spells. At level 10 it will be ~24 spells prepared including domain spells.
The issue is that even though these would be scattered across your spell level range, without more than a few per level bracket, the reality is that with up-casting and the way that Larian is currently struggling to implement it on their current system... you might only have 4 1st level spells in your first level window, but then you'll have the 3 2nd level spells you've prepared, PLUS the 4 1st levels spells being upcast to 2nd level, in the second button. the third button would then have the three or four 3rd level spells you've prepared, PLUS all of the 2nd and 1st level spells upcast to 3rd, etc. Your 9th level button would then contain all ~36 spells you have prepared that day, despite the fact that you only ever have one 9th level spell slot... it's a desperately inefficient way to do it. On top of that, many spells have casting options built into them, which are decisions you make when casting them; this kind of system cannot handle that as well.
I'd still say that Larian's current design is still worse, by a large margin.