For me, it's easier to think of Thaumaturgy like technology;
Killing spells:
1) If a sniper who has line of sight to a target (equivalent to a sympathetic link), a good high-powered rifle (focus equivalent), the right ammunition (focus equivalent), a high-quality scope (declaration), an automated range-finder (declaration), a good vantage point (declaration), compensated for elevation (assessment/maneuver), compensated for the wind (assessment/maneuver), compensated for air density (assessment/maneuver), compensated for the Earth's spin (assessment/maneuver), aimed at the target (maneuver), held his breath to get a steady hand (maneuver) can go and shoot someone dead in a single shot at three thousand yards I got no problem with it. Similarly with a wizard pulling off a killing spell; they just use different tools.
City-leveling blasts:
2) If an engineer who has coordinates for a target (equivalent to a sympathetic link), the right equipment and workspace for the job (focus equivalent), the right manuals and blueprints (focus equivalent), gets titanium alloy for the body (declaration), high-quality inertial GPS for guidance (declaration), graphite parts for propulsion exhaust (declaration), liquid oxygen/hydrogen tanks for fuel (declaration), the right electronics for the control systems (declaration), a high-quality impact trigger (declaration), gets high explosive for the initiator (declaration), neutron reflectors for the trigger (declaration), plutonium for the tamper (declaration), uranium for the first stage (declaration), tritium for the second stage (declaration), shapes the high explosive bricks into the right configuration to build the initiator (multiple maneuvers), shapes the plutonium into the tamper without killing himself (multiple maneuvers), enriches the uranium until it is usable (multiple maneuvers), shapes the uranium around the plutonium into a decent spark plug (multiple maneuvers), concentrates the tritium out of the base supplies and clears away any of it that decayed and positions it between the uranium and plutonium layers (multiple maneuvers), encodes the bomb's programming, navigation, control and evasion systems (multiple maneuvers), assembles all the individual components into a functional and perfectly measured whole (multiple maneuvers per component), pulls off the right subterfuge, stealth and deceit to avoid detection while doing all of the above (multiple maneuvers) and then successfully launches what he created after several years of work, I'd have no problem with him hitting New York with a ten-megaton nuclear missile. Because building a nuclear missile is very much doable given enough time, resources, physics expertise, crafting expertise, programming expertise, the wherewithal to actually do it and enough foresight and preparation to do it in secret or with enough backing that people don't stop you. It has literally been done tens of thousands of times by people all over the world with government backing.
Therefore, making a big spell that levels a city should be just as possible. Except that wizards with centuries of experience have far better skills in their craft than scientists and workers with a couple decades of training, and have had centuries to build up the right tools, procedures and knowledge to perform such a casting a good deal faster than their scientist colleagues - though scientists and engineers have the advantage of numbers.
Magic and Tech in general:
Neither have an upper limit. Not really. The upper limit in what tech can build is what resources declarations you can sink into it, what labor declarations you can use to construct it, and what knowledge/skill declarations you can do to design and pull off a functional concept in the first place. The upper limit in what magic can do is... pretty much the same. It just uses different skills than tech, and its tools and trappings are often intangible magical constructs rather than material supplies.
As mentioned above, it should be noted that a full Wizard has much higher skills and more powerful abilities than any equivalent mortal and thus can do it much faster. Teller and Uram, the designers of the hydrogen bomb, could at best pull off a +7 to +8 in aspects of Sholarship (+5 to +6 base, +2 in various different trappings via stunts) relevant to their work. A submerged thaumarugist can easily pull off +11 (+5 base, +2 specialization, +2 focus, +2 lawbreaker for warlock types or further +2 through higher refinement for non-warlocks). This means that a submerged wizard should be capable of pulling the same results a great deal faster than major scientists. It is only through sheer numbers and the general complacency of wizards that mortals are doing better than the magic guys.
On human sacrifice:
20.000 workers were killed during the construction of the Panama canal. Tens of thousands more of them suffered heavy injuries and were perpetually ill - i.e. had conditions inflicted upon them. That was less a direct contribution to the work than a willingness of themselves and/or their employers to go on with the work despite dangerous conditions. The work would have never been completed at the tech level we had back then without those sacrifices. In a more modern attempt there would be far fewer sacrifices but the individual workers would be far more skilled, providing declarations and assessments to the work through those skills, and they'd have far, far better equipment and resources (which comes again to declarations), and the engineers would have far better knowledge and understanding of the science required (which comes again to assessments and declarations).
Similarly for magic, you can either get skilled helpers to provide you with aid, more resources and higher skills to get the declarations required for the complexity, or you can choose to sacrifice people. Only the flavor changes; instead of the sacrifices meaning doing business despite harsh conditions and running your workers to the point of death in completing your work, you get metaphysical power from the sacrifice itself. The end mechanical result is pretty much the same, with the only differences being the ease with which wizards can get power from sacrifice... but the utterly corrupting nature of those sacrifices, not to mention their being far more illegal and pursuable by the various authorities than worker exploitation.
Ultimately, Thaumaturgy is about as problematic as technology. If you are having difficulties allowing a wizard knock down a building with thaumaturgy, you'd have just the same problems with your average mundane bad guy loading a van with a ton of gas and fertilizer and blowing it up. Both would take the same number of declarations game-wise. The tech way is a lot slower but doesn't require paying up multiple refresh for it, while the magic way is faster and more efficient but more costly in refresh and there are Laws and other metaphysics you have to contend with.
