Hello Kael and FFH Team.
Long Time FFH-Forum lurker and player of your Mod.
At first: Thank for creating the (imo) single best Fantasy-Strategy Game in existence (favorite Genre never cared much for RTS).
And still Improving / in Beta. At long last MoM has been unseated (Fond memories R.I.P.). Hats off to you, quite an achievement and still more to come. Cant wait.
This toppic / prospected change whould have such a big in impact that it has brought me to ditch beeing a Lurker and toss a few ideas into the mix as well.
First to the System in General:
While i think that the idea to unify the Summoner / Mage line is a very good one for making it easier for the AI as well as new Players and to keep the Tech tree simpler / easier to grasp, i do think that an oversimplification whould take out a lot of the fantsy feel and diversity / fun / strength the magic system offered until now.
And such a big part of the Mod as a whole.
I do think that both could be achieved at once in the following way:
While still unifying Summoning / Sorcery and the respective Techs into spellcasting as proposed:
(Tier 1 kept simple and easy / high Quality as known)
at Tier 2 Mages: each Civ (exept Dwarves / perhaps Divine for them / overlap surely possible if fitting) could have one (or more for Magic Heavy Civs) extra Spell (you allready said that this will likely happen on Civ-Level).
Since that level is quite early and easily achieved and so will be used far more frequently than Tier 3 by new Players. (Preferably in Spheres the Palace provides so they can be used right out of the box / are a natural Choice for inexperienced / new Players) so starters will find a nice, simple and fun selection of Spells at their disposal that are fitting for the civ and not an owerwhelming Array of mediocre seemingly indiferent Options as of now.
At Tier 3 all Spheres (Including Level 3 Divine) to offer 2! spells each at once instead of only one.
(to offer all the fun and diversity of the old System / freedom for your Ideas as Designers at a Level where flashy Effects with big impact are allright and Balance doesnt needs to be such a big restraint.
Also newer Players are not that likely to go that far down the tree before exploring the System at Level 2 first.)
Pleas leave us our Playthings.
In addition to give the Spellcasting Nations (any Nation with an Arcane, Summoning or Adaptive Leader since they can get arcane as well and it fits well with Kuoritates, Grigori and perhaps a bit less Malakim Style) a real edge in Magics,
instead of giving each civ the Option to build 6 (9 with 3 Liches) Archmage-Units you could make the UU Mages / Summoners of these Nations special Archmage Unique-Units
(so if i havent forgot any:
Sheim (Eater of Dreams, allredy exist),
Amurites (Wizards, allready exist),
Balspheraph (Puppeteer, allredy exist),
Ljosalfar (?),
Svartalfar (Illusionist, allready exist),
Luchirp (? You will come up with something good for sure

)
Grigori (? But they need em for sure without any Divines save Druids

),
Kuoritates (? Lamia...),
Malakim (?Perhaps a Divine Tier 3 whould be more fitting for them?))
Edit changed place: (special abilities for those could be implemented without risk for Balance at Tier 3)
(sorry for any misspellings of the civs / forgotten civs perhaps cant get them all from the top of my head)
and leave any other Civs with the normal national Unit Restriction for Archmages at 3 (6 with 3 Liches).
That Change would perhaps reduce the Codework as well since most of these allredy exist and the Limit of 3 needs not to be Changed then.
Calabim still whould have their fleshed out Melee-Caster UUs and i think don't need a re-work. (In addition to not being Arcane / Summoning / Adaptive) (on a side note: Haven't played those so dont know for sure if they will do fine without.)
That would prevent the not so magically inclined civs to go all out Magic nearly as well as the magic-heavy ones without a real advantage for them.
Comments to the spheres / cut spells in a separate Post later