But why use shards? The candles work just fine and are easier to get.
Also, mob heads, as most not-grinder mob farms give you so many that it's silly.
From what I've seen posted around, candles give the least boost to reducing instability and crystal clusters give the most with skulls somewhere in the middle.
Big problem I have at the moment is RNG is against me with netherrack (for fire shards). I'm not producing it fast enough from sludge, don't really want to waste redstone on making a batch. Might have to visit the nether.... Hmm, a stack of redstone is looking more appealing...
Why not use blood magic to get fire shards (or any other shard, really)? One shard and one smooth stone gets you one infused stone of whatever type the shard was which you can then break with a fortune 3 pick and you'll have a surplus of shards in practically no time! Granted, you need at least a tier 3 blood orb and I forget how much LP each process uses. You can even automate the whole process with the automated alchemy ritual, a couple of autonomous activators and an igneous extruder.