Thank you Karbock,
your (B) formula seems perfect because I download albums with Deemix and they have ALL disc number even if it's a single disc, as 1/1.
So Either I disable that tag for all albums included multiple ones, or I get it for all albums.
But while your formula does indeed separate only albums with 2+ discs into CD1/CD2, it overwrites all tracks of all copied albums to a single .mp3 track with no title.
Basically what I get in destination device is \Artist - year - Album\ containing 3 files:
.mp3
.lrc
folder.jpg
I don't understand what's missing. This doesn't happen with my formula, tracks are not sorted by CD, but they all have their track title.
EDIT
Ok, it seems it needed explicit track title as well, like
<Genre Folders>\<Album Artist>\<Album Artist> - <Year (yyyy)> - <Album> [<Grouping>] [<Bitrate>]\$IsNull(<Disc Count>,,$If(<Disc Count>=1,,CD<Disc#>\))<Track#> - <Title>
This way tracks do not overwrite, but I still don't understand why I didn't need the last part of the formula in my previous version.
Anyway this seem to be (partially) SOLVED
Thank you so much!
EDIT2
Oh, wait!
What about playlist.m3u?
How do I copy those to each album as well?