The original statement you were responding to was :
Now, 'impossible' is a strong word, but it's true that research costs CGs, and if you focus heavily on research you will need a solid CG production sooner or later. It looks like your definition of "tech rush" only involves the start of the game, whereas @ImaTomato appears to be looking at least at mid game levels of research:
When you look at an extremely early state of tech rush, then of course it isn't hard to cover most of your CG needs with purchases and other means. That is what you are looking at.
But when you consider the state of a tech-rushing empire around 2040 or later, then I'm fully with @ImaTomato: none of the steps you suggested will provide a meaningful amount of CGs at that time - you actually need to produce most of them. That is what @ImaTomato sees.
I'm sorry, but this is really grasping at straws. Not much about tech in 2040 can really be considered a "rush". By that point so many variables exist that talking about something this specific is nearly impossible. How far did you expand? How rich is your space? How many colonies do you have? Did you complete your precursor? Which one was it? How many minor artifacts did you get? Were you attacked early? Did you conquer several AIs? Even using the exact same empire build, positions 40 years in will vary wildly depending on these factors.
The point of tech rushing is that it's very consistent - if you do it correctly, you can be successful regardless of your spawn position, space, planet count, etc. But what the strategy does is provide quick tech-based solutions to dealing with each of these variables - not an exact blueprint for the entire game. Quite often, the best play while tech rushing is actually not to continue ramping up tech and cg into the 2030s and 40s, but to start producing a ton of alloys and prepare for conquest well before then. In these cases, what sense does it make to talk about needing "a massive CG industry" for tech rushing?
In fact, in basically every other example I can think of, the same thing holds true - even in fanatic pacifist tech-focused ones. You actually just don't need a massive CG industry... ever. Artisans become more and more efficient as the game goes on and you specialize your economy, and the ratio of artisans to researchers needed remains low. Once the galactic market opens up, you can even start buying a large percentage of your CGs again!
Both of you are right within the frame that each of you is looking at. But given that tech rush doesn't simply end by 2020, I'd say that @ImaTomato has a point, and you havent answered it.
My original reply clearly addresses this:
"Now don't get me wrong, I don't actually think this is a real problem. After all, you can only buy so many CG before prices tank; you have to transition into producing them eventually."