I'd like to seem some sample numbers, but my first impression is that your idea doesn't scale that well. Of course, we might also be disagreeing about what qualifies as 'big thaumaturgy'.
Here's my take:
power level 3-8. (characters of around 5-6 refresh)
This is what amateurs can manage. So, for example, when the Paranet wards your house, this is probably what you end up with. For more powerful people, they do this level of thaumaturgy because it's easier and more familiar (to them) than the mundane equivalent.
Power level 12-18 (characters of around 7-8 refresh)
This is probably the maximum power level manageable by 'powerful' non council members. So it's what we'd expect from (young) warlocks, people who are powerful, but don't have the depth of power to be considered for the council, etc.
Power level 25-30 (characters of around 10-12 refresh)
This power level is what even the most junior members of the white council can manage. Someone who is 'unaffiliated' being able to do it would be highly surprising, and if they manage it, it's usually because they are killing someone to do it, or similar bad stuff.
Power ~40-60 (characters of around 20-25 refresh)
This is the rough limit of the vast majority of white council members.
While feats on this scale are 'impressive' they generally do not threaten the status quo of the world outright. Leveling buildings/city blocks, making Frankensteins monsters, creating large mansions out of grass clippings, stuff like that.
Power ~80-~120 (characters of around 30-40 refresh)
This the the realm of senior council members: destroying cities, making new types of vampires, calling up godzilla, etc.
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Next, I don't like the implication that big thaumaturgy needs 'some external source'. This seems to turn it into a game of 'mother may I'. This means that any time the GM provides a problem that big thaumaturgy can solve, it's not solvable unless he also provides the 'extra resources' that big thaumaturgy would require.
Hmm, starting a new thread, with a different but related discussion.