Personally I wouldn't use any epics as food. It's much easier to make 4* chickens than it is to get epic champions and even the weaker ones might get buffed someday, it has happened before. They are also useful for Faction Guardians. I would get Apothecary and the rest of my Demon Lord Clan Boss team up to level 60 and then take them all into Minotaur for masteries. Don't bother farming Minotaur till your team can do level 15 consistently.
I agree that buying shards for Shamael isn't worth it. Even if you get the top chest from the hardest difficulty of Hydra it will still take a really long time to get Mithrala and it's not too hard to get the easiest chest.
A few rares you might try holding onto unless you absolutely need the space;
-Coffin Smasher, really good for Demon Lord and can solo Frost Spider
-Odachi isn't OP or anything but could be good to round out a Shadowkin Faction Wars team
-Ironclad can be useful for crowd control particularly in Orc Faction Wars and is generally just kind of fun to play around with
-Fleshmonger's decrease defense ability will make Skinwalker Faction Wars a little less painful
-Abyssal is another Faction Wars champ
-Master Butcher can be really good for Bommal
If you don't have an unkillable Clan Boss comp then I would definitely build Coffin Smasher, as for the rest I'd just hold onto them and build if/when I needed them to clear specific content. I think it's fun to collect all of the starter champions and personally I find Elhain to be super useful all over the place. She's easy to build for damage since she doesn't need accuracy, great for clearing waves and every skill she has is a double hitter for Fire Knight or killing Rotos, also sometimes you want to deal damage without causing additional effects either to avoid filling up a debuff bar or so you don't trigger boss mechanics.
ETA: Don't feed any rares that have that blue swirly logo at the center bottom, those are used for fusions you'll want to do at some point. Some are actually pretty decent champions, too.