The nuclear process itself doesn’t produce co2, the construction of the building does, you are absolutely right about that.
This goes for all concrete needed for renewables as well, massive hydro power dams will produce far more co2 during construction than a nuclear powerplant.
It is obvious that the economixs have changed in 30 years, and they will change in the next 30 years as well. The hesitation of building new nuclear powerplants will not make the situation better. The best time to build nuclear powerplats was perhaps 30 years ago, the second best time to build them is today.
By using economics as an argument you are deliberately advocating against using all tools to reduce global warming.
Base load absolutely exists, without it our society would fall apart.
Nuclear power would give us time to reduce the baseload to managable levels and further develop renewables so they can cope and we can transition away from coal power that needs kilometer long trains of coal every day, to me that sounds like it is worth paying a bit extra to do it faster than drag our feet when we have the knowledge and capability to do it.
I bet that in 30 years when this debate is still going on, you will say that we should have started building nuclear plants 30 year ago because the economics has changed since then.