you can get the number of tracks per album with
ItemCount(AlbumKey/(/))
as for summing and dividing, the only way to do that is with global variables. it's a fun space to play in but also not intuitive. if you make a new smartlist and past this into the "import/export"
[=save(0,var_number_of_tracks[album artist (auto)][album])1]=1 [=save(0,var_album_year_sum[album artist (auto)][album])1]=1 [=save(math(1+load(var_number_of_tracks[album artist (auto)][album])),var_number_of_tracks[album artist (auto)][album])1]=1 [=save(math([Year]+load(var_album_year_sum[album artist (auto)][album])),var_album_year_sum[album artist (auto)][album])1]=1 [=save(math(load(var_album_year_sum[album artist (auto)][album]) / load(var_number_of_tracks[album artist (auto)][album])),var_album_year[album artist (auto)][album])1]=1 [Media Type]=[Audio] ~sort=[Album DR Average],[Filename],[Artist],[Media Type],[Album Artist (auto)],[Album],[Disc #],[Track #]
then, add an expression column to the smartlist and paste this:
formatnumber(load(var_album_year[album artist (auto)][album]),0)
the result should be what you are looking for.....