There are a few things I feel need to be considered with respect to the (very high) suggested number of shifts possible in a thaumaturgy ritual.
From the novel Storm Front, and as mentioned in YS301, The "Heart exploding spell" had a complexity of 36+, and in order for Victor Sells to successfully cast it, he had to do substantial prep work, as well as animal sacrifice and harness energy (inflict Consequences on) his wife and the two Becketts. He also had to take a number of Consequences himself in order to harness the power required, and was also dependent on being able to tap into a thunderstorm.
Harry told Murphy that he himself could summon the power required to cast the spell, but it would kill him to do so, without others helping.
As mentioned in Changes:
The 'larger' version of the heart-exploding spell was the Bloodline Curse that the Duchess Arianna was preparing at Chichen Itza required several days of preparation, with a number of individuals participating (if not hundreds) as well as a number of human sacrifices. Enough for blood to pool in some areas of the pyramid. With the power levels of the principal casters involved (high ranking Red Court nobles/borderline Old Gods), the sheer number of participants available to absorb consequences (hundreds or more), the ready use of human sacrifices (each one providing ~20 shifts) and the use of a Place of Power, this ritual still wasn't ready in a day.
Now, for any spell greater in complexity than the caster's Lore, which means anything using 6+ shifts of power requires the use of Preparation involving Aspects, Scene skipping, Consequences or mini-Scenes where relevant skills are successfully used in a manner which could work towards reaching the complexity of the spell. Lastly, the power of the spell needs to collected and controlled successfully using Conviction and Discipline. The 'slow but safe' method of doing so, would be for the wizard to collect/control a number of shifts equal to or less than the lower of his Conviction or Discipline -2, assuming that would provide at least 1 shift of power is collected per exchange. This would allow a wizard to avoid taking Mental stress from summoning the power, as well as not triggering Backlash or Fallout. If we were to assuming that the wizard is only able to manage 1 shift of power per exchange safely (by either only having Conviction of Average:1 or Discipline of Good:3) then the Heart exploding spell in Storm Front would take 36 exchanges to collect the power needed, unless there were power boosts available from other casters, focus items, etc. Now, while an exchange is a Combat measure of time, a Scene (YS314) could be considered the non-combat equivalent and doesn't have a specifically assigned length of 'story time' it can range from a minutes to a half hour or more. Assuming that each Scene spent summoning power takes ~15 minutes, then a Wizard operating on their own would need 9 hours without interuption, to cast the 'Heart exploding spell'. A significantly more complex spell, that uses 100+ shifts of power, would take the same wizard an entire day (24 hours) or more to summon and cast the power. Again, this would also have to occur without interuption, which means that the wizard would start running into the limits of human endurance. Attempting to push past that would trigger (with me as GM) pretty much automatic Stress on the character, and/or Consequences, and/or free Compels. In the case of a single wizard attempting a 500+ shift spell, that would pretty much be an automatic death sentence for the character in this case, since the character would have been casting (again, no interruptions for minor, unimportant things like food or sleep) for 125 hours/5+ days and automatically the character would have started getting Consequences like Weary, Hungry, Headache, etc which would get Tagged to cause an automatic failure with summonging/controlling the power at some point, with the character then being hit with either Backlash or Fallout, and no character would be able to take 100+ shifts of Backlash or Fallout.
I think you are miscalculating the required shifts for yout example;
The Bloodline Curse killed thousands of individuals all over the world. Assuming that we use +2 complexity per extra zone instead of spreading the spell's effect between targets (which would require a lot more power) we are still looking at thousands upon thousands of shifts of power. Assuming that the Red Court had about 10.000 vampires and infected worldwide (roughly outnumbering wizards of the White Council 3 to 1 or so), we are looking at 20.000 shifts as an absolute minimum. Assuming an exchange is half a minute (see below), we are looking at a week of casting, minimum, by a weak inhuman caster, or 3 days by a strong inhuman caster.
Secondly, an exchange is not only a measure of time for physical combat; exchanges exist in non-physical activities just fine. Any type of conflicts use exchanges, from social to mental, to physical, to using your skills for a set task out of combat. Casting spells specifically uses exchanges, not scenes. Besides, there is no reason a thaumaturgy would take a dozen times longer to cast out of combet than inside combat and we have seen Harry cast 10-shift thaumaturgies (with his discipline of 3) in short enough time that a running villain doesn't lose him and a demon breaking through his doors doesn't have enough time to reach him. And even 25-30 shift spells when cast by a two-bit sorceress (i.e. entropy curse in Blood Rites) only take about 5 minutes.
Third, we have seen single wizards cast nuke-equivalent spells in a couple of hours. Prime example is Cowl's Darkhallow. Even if he has a +7 discipline and conviction with a +8 necromancy control and +4 necromancy power bonus (not unusual for the greatest necromancer of his time - especially given that Necromancy control can substitute for evocation control with Kemmlerian Necromancy) and thus can summon 11 shifts of power with absolute safety, by your counting the Darkhallow could not have been more than 150 shifts of power. That is enough to level two-three dozen buildings but not nearly enough to kill everyone within miles as a mere side effect or turn one into a minor god. By my count on the other hand, the Darkhallow could have well been 4500 shifts of power - about the right power level for a spell of its apparent magnitude (though the slaying of thousands of humans and eating of hundreds of spirits as the first spell level was completed would probably jump the ritual's power by a factor of ten upon completion)