You're missing several things.
- City districts house 5 pops by default, you can get +1 from a tech and +1 from a tradition.
- City districts cost 500 minerals, resource districts cost only 300. (which would get you 2 research stations, for instance)
- City districts take 2x longer to build than resource districts. (480 days vs 240 days)
- City districts have 2 energy upkeep, resource districts have only 1 energy upkeep
- City district + Commercial Zone combo that you're proposing costs both 1 district slot AND 1 building slot.
Just taking a generator district only costs 1 district slot, leaving the building slot available for something else.
- Commercial zones take 1 year to build (unimportant), costs 300 minerals (important), and require 2 energy upkeep (important).
- Technicians is a more "resource dense" job than clerks in terms of maximum energy output (4 energy/job vs 2 trade -> 2 energy/job)
This is particularly important because pop growth is the biggest limiting factor to economic expansion until perhaps very late game.
- That theoretical strategy needs more pops, which means more food and more consumer good expenses to maintain a similar energy level.
I presume by "building the buildings for it" you mean the Energy Grid/Nexus? Lv.1 is +15% energy credits, and Lv.2 is +25% energy credits from techies. Neither of those would add up to 9.6 energy credits from gennies, though.
It's not possible for me to do a super in-depth analysis of this in the span of 15 minutes, but here's a very rudimentary one:
Energy districts has an upfront cost of 300 minerals, 1 district slot and 2 pops for +8 energy, for a net monthly resource transaction of +7 energy, -2 popsworth of maintenance (food and consumer goods) (which varies by living standard, chosen food policies, etc).
The proposed city-commerce combo has an upfront cost of 800 minerals, 1 district slot, 1 building slot and 6 pops for +12 trade value, for a net monthly resource transaction of +12 trade value, -4 energy, -6 popsworth of maintenance.
If trade is set to Wealth, that becomes a net resource transaction of +8 energy, -6 popsworth of maintenance.
The energy district route is more than 2x cheaper to set up, requires 3x less pops to fully man and so requires 3x less time to outfit, and has a 3x lower maintenance cost due to simply having 3x less pops to man, which means less supporting infrastructure needed to offset these costs (Consumer goods, Farms), which
themselves require some energy input, mineral input, pop labor input, in addition to upfront costs of minerals, district slots and building slots.
Obviously the analysis is pretty limited (doesn't take into account limited Tech districts, the Stock Exchange building, the Energy Grids/Nexii, or effects of amenities), but it should be clear that cities doesn't handily beat generators in energy production.
Trade value is increased by stability.