Then that makes multi-circle Sorcerer Lords instantly more powerful than single circle Lords. Still a VERY bad idea.
Not really. It only makes a few more spells available. Single-circle lords are still much more powerful in the long run thanks to being able to get the most powerful spells.
That may be something to consider but please let's not get caught up in the "balance" obsession.
It's already killing another "spiritual successor" (Pillars of Eternity, that's trying to reinvent the wheel - a "balanced" wheel actually - with disastrous results).
Remember that:
1. It's a SINGLE player game
2. MoM never was balanced really
3. You might choose a "weaker" sorceror on purpose to have a different/more challenging game
Eh. A single-Circle Sorcerer will have access to much more powerful spells, and considerably more of them. A multi-Circle mage will have (slightly) increased flexibility.
Take the case of a 4/4 Element/Effect split. That Sorcerer Lord will have 56 spells in total. Of those, one will be tier 5, one will be tier 6, one will be tier 7, and one will be tier 8.
A Sorcerer Lord instead doing 8 in a single Circle will have 52 spells. Five will be tier 5, four will be tier 6, four will be tier 7, and three will be tier 8.
Also consider that, if what I'm reading is accurate, Tier 9 spells will be restricted solely to those who invest nine full picks into a given realm, and even then it's 50/50 which one you'll get unless you also pick Mastery.
So, the multi-Circle gets a few more spells, and knows for sure what his/her tier 5/6/7/8 spells will be, but won't get any other 5/6/7/8 spells (in this specific example). The single-Circle gets slightly fewer spells, but has more of them at higher level. He/she has slightly less certainty about any given spell that is tier 5 or above (as he/she is getting all of them within that Element or Effect up to tier 4, anyway), but is still running a better than 50% chance for any given spell even at tier 8 (again, in this specific example).
It seems to even out pretty well to me. The only cases where a multi-Circle Sorcerer is going to completely overpower the single-Circle is in the case of the multi-Circle one having several Circles at quite decent ranks--4/6/4 or 8/8 (maybe, depending on the realms chosen) or some other combination where the multi-Circle Sorcerer is just clearly out-studying the single-Circle. But then, if one is so predisposed to load up on the negative disciplines required to get the extra points to double up on Circles deeply like that, then, ostensibly, one has other drawbacks to account for it. Whether or not that is true will have more to do with the potency of the drawbacks in question, but I'm not certain that anyone can really argue that if a Sorcerer is going to be focused enough to go deep on multiple realms that he/she shouldn't have quite an advantage for doing so. It's also worth noting that while such a Sorcerer Lord may have a much larger potential pool, he/she will still have to actually research all of those, and that's a lot of resources to devote to such a task.