Speaking of overchannel, does the necrotic damage kick in if you switch up spells? If you maximize fireball in round one and the maximize lightning bolt (for the first time that day, wrt lightning bolt) in round two, do you suffer necrotic damage because you overchannelled twice or do you avoid necrotic damage because you only overchannelled lightning bolt once?
Thinking about the cantrips, I think now I would not go 1/2 of a 1st level spell directly because then you get 1/2 of 3d12 the third time you cast it. Instead, I would switch the die type so that cantrips that are overchannel the second time would do 2d6, then 3d6, 4d6, etc. But that is a house rule, of course. As written, it is no damage and you could overchannel ray of frost all day long without fear of necrotic damage. But maybe that is to counter the likely lesser power of evokers vis-a-vis other types of mages (at least in 3.x, evokers were a weak specialization, as damage was not where it was at - don't know about 5e though).