generally me likey
the numbers involved are obviously sketchy at this point, however from appearances it looks like the negatives could start outwaying the generation very quickly. if you have a sov and a shard and you imbue a channeler and summon a beastie, then you'll already be in a no-generation scenario, which is messed up. i can see the logic in mana maint for summons as it requires energy to bind them to the material plane, but channelers are just supposed to be conduits. i get that you need a mechanic to limit imbuing, but this is not it.
second problem without a personal mana cap you have no limitation on who can cast more powerful spells. literally any level 1 channeler will be able to cast flaming-suns-from-hell.
i think you can fix both by looking at stats. my POV was always that by imbuing you sacrifice personal (rather than global) power for greater strategic flexibility, like sauron and the rings. if you use INT to limit what spells a character can understand, and then WIS/ESS to determine the power of spells, then imbuing reduces the sovs spell power without risking negative generation. so more channelers = individually weaker. and there is therefor no need for separate mana pools.
the next question is what determines you ultimate mana pool size? targetting this might be better than piling on loads of mana maintenance. ie, every summon reduces the point to which your global mana can regenerate.
i'm also going to join those speaking out against the automatic shards for everyone. if you play a large map with 10 factions that's 10 shards already. add half as many again so that you can get more shards without conquering a civ, then you've already got enough for four shards of each type. that's way too many and cheapens the concept. i can see it for some (maybe half) of starting locations, but not all. if you want to give people a bassline bonus to regen, then add some level 2, one per faction building that gives one point of extra regen.
i think the starting positions need a lot of attention. i'm always restarting games (often after about an hour in) because i've got a sucky starting location. there needs to be a more balanced formula like each faction getting
1 food resource
1 other resource (material, gold or metal)
1 special resource (horses, a shard, lost library etc)
this adds a lot more consistency, will hopefully help balance and give small civs who don't get the opportunity to expand a fighting chance.
finally i'm going to argue again that spell books should be split up by purpose rather than elements. making the starting books element based but the researched ones purpose based is inconsistent and you end up with fire spells that aren't in the book of fire, or powerfull non elemental spells making the elements redudant.
if you replaced the starting books with a book of lesser summoning, book of lesser destruction and so forth, and had each of those contain spells of all elemental types, then we wouldn't have this stupid situation where everyone feels that they have to take them all just in case they end up starting near that shard, but at the same time you only need one because they all work the same way. this way choosing spellbooks at character creation would actually be a meaningfull choice (that could be taken by ai sovs as well as user ones) instead of a source of confusion.