Implement a system where higher level disciplines require a min number of shards to cast. Shards must be of that element. For example:
1. Fire element
level 1 = 0 shards (burning hands)
level 2 = 1 shard (flame dart)
level 3 = 2 shards (fireball)
level 4 = 3 shards (whatever comes next, is it firestorm?)
If you are a fire Archmage with only one fire shard, you can only cast level2 spells (Fire Disciple, I think its called), even though you could be casting level 4 spells if you had three fire shards.
This also allows you to remove the 'casting time' requirements from tactical battles (https://forums.elementalgame.com/424177). Instead of trying to balance magic using a casting time mechanic, you'd use the same shard system outlined above for both overland and tactical spells.
Edit: Tech and/or spells could turn shards from one type to another.