It seems to me that the number of shards given when sacrificing creatures is unbalanced toward creatures with a high number of levels. For example I have a maxed untokened Shmsh (~7 levels) who gives 1792 for its level. I also have a maxed untokened Imp. Aramor (~3 levels) who gives 28 for his level. For a final example I have a maxed tokened (13 tokens) GG Drachorn (~5 levels) who gives 1160 for his level and tokens.

As it works now it looks to me like a creature's level is rather overvalued but only when it has more than 5 levels, meaning certain creatures are very easy to farm shards for, while other ones are much more difficult.

Yeah I think the other values could be boosted a bit, specifically age should matter a lot. Age and level are correlated strongly for most creatures although level stops being relevant once it is maxed, age is something you always want on a creature and is more natural than tokens or heat which can be easily added/removed.